Geomorphology 266 (2016) 20–32

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Geomorphology

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Geomorphic evolution of Dehra Dun, NW Himalaya: Tectonics andclimatic coupling

Swati Sinha, Rajiv Sinha ⁎Department of Earth Sciences, Indian Institute of Technology Kanpur, Kanpur 208016, India

⁎ Corresponding author.E-mail address: rsinha@iitk.ac.in (R. Sinha).

http://dx.doi.org/10.1016/j.geomorph.2016.05.0020169-555X/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 November 2015Received in revised form 1 May 2016Accepted 2 May 2016Available online 6 May 2016

The Dehra Dun is a good example of a piggyback basin formed from the growth of the Siwalik hills. Two large riv-ers, the Ganga and the Yamuna, and their tributaries deposit a significant part of their sediment load in the Dunbefore they enter the Gangetic plains. This work documents the geomorphic complexities and landform evolu-tion of the Dehra Dun through geomorphicmapping and chronostratigraphic investigation of the incised fan sec-tions. Lesser Himalayan hills, inner and outer dissected hills, isolated hills, proximal fan, distal fan, dip slope unit,floodplains, and terraces are themajor geomorphic units identified in the area. Isolated hills of fanmaterial (IHF),proximal fan (PF), and distal fan (DF) are identified as fan surfaces from north to south of the valley. The OSLbased chronology of the fan sediments suggests that the IHF is the oldest fan consisting of debris flow depositswith a maximum age of ~43 ka coinciding with the precipitation minima. The proximal fan consisting of sheetflow deposits represents the second phase of aggradation between 34 and 21 ka caused by shifting of depositionlocus downstream triggered by high sediment supply that exceeded the transport capacity. The distal fan wasformed by braided river deposits during 20–11 ka coinciding with the deglacial period. The IHF, PF and DF sur-faces were abandoned by distinct incision phases during ~40–35, ~20–17, and ~11–4 ka respectively. A minorphase of terrace deposition in Dehra Dun was documented during 3–2 ka. Our results thus show that the evolu-tionary history of the alluvial fans in Dehra Dun was primarily controlled by climatic forcing with tectonicsplaying a minimum role in terms of providing accommodation space and sediment production.

© 2016 Elsevier B.V. All rights reserved.

Keywords:Intermontane valleysHimalayan forelandValley fillsFan deposits

1. Introduction

Tectonic and climatic processes have interacted through tightlycoupled feedbacks at variable spatiotemporal scales to shapemountain-ous landscapes along the Himalayan belt (Seeber and Gornitz, 1983; Oriand Friend, 1984; Lavé, and Avouac, 2000, 2001; Kirby and Whipple,2001; Kumar et al., 2007; Suresh et al., 2007; Singh and Tandon, 2007,2008). Intermontane valleys (Duns) are important elements of moun-tainous landscapes in the Himalaya that have developed in responseto climatic fluctuations and neotectonic activities during theQuaternaryperiod (Kimura, 1999; Singh et al., 2001; Dühnforth et al., 2007;Goswami and Pant, 2007; Kumar et al., 2007; Suresh et al., 2007;Singh and Tandon, 2008; Suresh and Kumar, 2009; Malik et al., 2010;Dutta et al., 2012). Alluvial fans, fluvial terraces and floodplains havebeen mapped as the main landforms in the Duns (Nossin, 1971;Nakata, 1972; Rao, 1978; Chandel and Singh, 2000; Singh et al., 2001;Suresh et al., 2007; Thakur et al., 2007; Singh and Tandon, 2010; Sinhaet al., 2010; Dutta et al., 2012). The alluvial fans in the Duns occur atmultiple topographic levels (Nakata, 1972; Suresh et al., 2002, 2007;Philip et al., 2009) and have been described as a piggyback system

developed through multiple phases of aggradation and entrenchmentduring the late Quaternary period (Kumar et al., 2007). The develop-ment of these fans is influenced by tectonic and climate factors suchas glacial-interglacial cycles, base-level changes, sediment supply fromthe catchment, availability of accommodation space, and degree of tec-tonic activity (Harvey, 1990, 2004; Silva et al., 1992; Ritter et al., 1995;Harvey et al., 1999a, 1999b; Harvey and Wells, 2003; Mather andStokes, 2003; Mather et al., 2009; Viseras et al., 2003; Robustelli et al.,2009). Fluvial terraces also occur widely in Duns, and multiple levelsof terraces have been mapped in the frontal parts of the Himalaya,e.g., three to five levels in the Yamuna valley (Singh et al., 2001; Duttaet al., 2012) and four levels in the Ganga valley (Sinha et al., 2010).Most rivers draining the fan surfaces are incised and therefore have de-veloped rather narrow floodplains except in the distal parts (discussedlater). In a more recent work, several piggyback basins along the Hima-layan front were examined to explore storage and release of sedimentsfrom these basins and their downstream influence (Densmore et al.,2015). The suggestion has been made that the Duns have controlledthe temporal variations in sediment flux into the Gangetic plains andhave therefore influenced the landscape development in the plains(Densmore et al., 2015).

Despite several generations of geomorphic mapping and landformevolution studies in the Duns in the NW Himalaya, we note significant

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gaps in terms of terminologies, field validation, and controlling factors.For example, the term ‘piedmont’was used to describe the depositionalsurfaces by Singh et al. (2001), Thakur and Pandey (2004) and Thakuret al. (2007) but they do not ascribe the processes that might haveshaped them. Generally, the terminology ‘piedmont’ is used for erosion-al and depositional landforms formed adjacent to a mountain front(Leece, 1990; Goudie, 2004). Chandel and Singh (2000) described thepost-Siwalik deposits in Dehra Dun as ‘doon piedmont’ but withoutmuch process interpretation of the associated geomorphic elements. Fur-ther, a suggestion was made that the geomorphic evolution of the Duninvolved a number of depositional and erosional phases superimposedon each other and regulated by climatic changes and tectonics (Nakata,1972), but very limited information is available in terms of field evidenceand chronological data. Age data on different geomorphic surfaces in theDuns has been limited, and therefore a sound chronostratigraphic frame-work of fans is lacking. In summary, the understanding of the evolution oflandforms and their spatial relationships is fragmentary.

Wepresent here a detailed geomorphologyof theDehraDunbased onmapping from high-resolution satellite images and digital elevationmodels (DEM) coupled with field validation. Stratigraphic framework isbased on several lithologs from incised fan sections and a sound chrono-logical data that provides an insight to the depositional history of theDun.

2. Study area and previous work

The Dehra Dun is one of the largest intermontane valleys betweenthe Mussoorie Ranges of the Lesser Himalaya to the north and theSiwalik Ranges to the south (Fig. 1). The Siwalik Ranges are mainly agroup of rocks and categorized as lower (mudstone), middle (sand-stone), and upper (conglomerate) units. The Dehra Dun is bounded bymajor faults from all sides; the Main Boundary Thrust (MBT) to itsnorth, the Himalayan Frontal Thrust (HFT) to its south, the YamunaTear Fault (YTF) to the west, and the Ganga Tear Fault (GTF) to theeast, making it a structurally isolated block (Fig. 1). The Ganga and Ya-muna rivers break through the Siwalik Ranges along the transversefaults, the GTF, and the YTF, respectively (Karunakaran and Rao, 1979;Raiverman et al., 1994). The NW-SE trending intermontane valley is~80 km long and ~25 kmwide. It is an asymmetrical valley with a gen-tler (0–5°) SW slope and a steeper (0–10°) NE slope. Alluvial fans,

Fig. 1. (A) Locationmap of the Dehra Dun. (B) Study area including thewestern part of the Dehcoloured arrows indicateflowdirections of the rivers. TheMainBoundary thrust (MBT) and thein the field while Bhauwala thrust (BT), Majahaun fault (MJF), Asan, and Tons fault are shown

hillocks, river terraces, and floodplains are the major geomorphicunits in this region, and the valley fills have been described as ‘Dungravels’ (Nossin, 1971; Nakata, 1972; Singh et al., 2001; Thakur andPandey, 2004; Thakur et al., 2007). The study area is confined to thewestern part of the Dehra Dun between latitudes 30°20′ to 30°35′ Nand longitudes 77°40′ to 78°05′ E bounded by the Tons and Yamuna riv-ers to the east andwest, respectively (Fig. 1). Several ephemeral streamsnamely, the Suarna, Sitla Rao, Koti, Mauti, Chor Khala, and Nimi, areflowing in this area. These streams and several other smaller rivuletsflow SW and join the axial Asan river that in turn flows NW andmeets the Yamuna River. The Siwalik hills to the north of the valleyare highly dissected and disjointed and are separated by topographicalbreaks that are marked by sharp turns of the streams flowing throughthe area (Thakur et al., 2007). The southern part of the valley is thedip-controlled slope of Siwalik Ranges and mainly composed of upperSiwalik conglomerates and the boulders and gravels of quartzite andsandstone derived from the middle Siwalik hills (Thakur et al., 2007).

Table 1 shows the various geomorphic elements in the Dehra Dunarea proposed by various workers. Nossin (1971) mapped several large-sized fans descending from the foothills of the Mussoorie ranges. Nakata(1972, 1975) identified four fill surfaces and interpreted them to have re-sulted fromdifferential uplift within the valley. Chandel and Singh (2000)mapped Siwalik hills and post-Siwalik deposits as piedmonts that werefurther divided into high-level terraces, and upper and lower doon pied-monts. Alluvial fans are the major geomorphological units identified bySingh et al. (2001) and Thakur et al. (2007) in the Dehra Dun. The MainBoundary thrust is the major structure that separates the Siwaliks andthe Lesser Himalaya. Other major structural features documented in thisarea include the Santaurgarh thrust (Raiverman et al., 1983), theBhauwala thrust (Singh, 1998), the Asan and Tons faults (Thakur andPandey, 2004), and the Majahaun fault (Thakur et al., 2007).

Fan deposits in Dehra Dun mainly consist of boulders, cobbles, andpebbleswith a sandy and siltymatrix (Nossin, 1971; Thakur, 1995). Pre-vious workers (Singh et al., 2001; Thakur et al., 2007) have suggestedfour major depositional units in the valley based on OSL ages. Unit A isthe oldestwith ages varying from40.3±3.9 to 33.6±4.7 ka andmainlyconsisting of poorly to fairly consolidated gravels with interlayeredmudstone and sand beds (Thakur et al., 2007). Unit B of unconsolidatedmassive gravels overlies unconformably on Unit A and yields maximum

ra Dun valley bounded by the Tons River to the east and the Yamuna River to the west. RedSantaurgarh thrust (ST) are shownwith thick black lines as their activity has been observedwith a dotted black line, as their activity has not been observed in the field.

Table 1Schemes of geomorphic mapping by different authors in the Dehra Dun area.

Authors Major geomorphic units

Nossin (1971) 1) Doon fans: high level—upper and lower group; 2) Doonfans: principal level—dissected and undissected surface;3) Doon fans: lower realm of coalescence; 4) Doon fans:terraces below principal level

Nakata(1972, 1975)

1) Upper Dun surface, 2) Middle Dun surface, 3) Lower Dunsurface and higher river terraces, and 4) Middle river terraces

Chandel and Singh(2000)

1) Lesser Himalaya (~2500 m), 2) dissected Siwalik hills(~900 m), 3) pedimented Siwaliks with thick gravel cover(~750–450 m), 4) isolated N\\S trending Siwalik hills(~850 m), 4) piedmont (upper and lower piedmonts)

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and minimum OSL ages of 29.4 ± 1.7 ka (Singh et al., 2001) and 20.5 ±1.8 ka (Thakur et al., 2007), respectively. Unit C has been dated between~22.8 and ~10.7 ka and is mainly composed of angular to subangularpebbles and gravels. Unit D has been dated as ~30 ka and covers thenorthern slopes of the outer Siwalik range at the southern part of thevalley. The total thickness of fan deposits in the Dun using tube-welldepths, is estimated to be ~100 m in the proximal part and N300 m inthe distal part (Thakur et al., 2007). The gravel units in fan depositsaremassive to thickly bedded, and poorly consolidated to unconsolidat-ed. Clasts are randomly oriented and angular to subrounded (Singhet al., 2001; Thakur et al., 2007).

3. Data and methods

Geomorphic mapping of the Dehra Dun valley was carried out usingsatellite images, elevation data, and topographic maps. Satellite dataconsisted of IRS P6 LISS 3, LANDSAT image, Shuttle Radar TopographyModel (SRTM) version 4 (spatial resolution 90 m), and AdvancedSpaceborne Thermal Emission and Reflection Radiometer (ASTER) v. 1(spatial resolution 30 m). The IRS P6 LISS 3 (spatial resolution 23.5 m)

Fig. 2. Geomorphic map of the Dehra Dun. The geological structures marked by the previouST—Santaurgarh thrust which are confirmed through field evidence. F2 represents MJF—Masupported by any field evidence. Different geomorphic units and features are described as CBhills, IHF—Isolated hills of fanmaterial, IHS—Isolated hills of Siwalik rocks, PF—Proximal fan, DF—

images were provided by NRSC (National Remote Sensing Center,India) and the rest of the data were downloaded from the USGS website (http://earthexplorer.usgs.gov). The LANDSAT (resolution 30 m)and SPOT (resolution 5 m) images were fused together in order to en-hance the spectral and spatial properties. Major geomorphic featureswere mapped using the LISS 3 image, fusion image of LANDSAT andSPOT, topographic maps of 1:50,000 scale and the ASTER DEM. TheArcGIS 10 software was used for mapping and geomorphic analysis.The ERDAS imagine 8.5 software was used for image processing andRiver Tools v. 2.4, and ‘System for Automated Geo Scientific Analyses’(SAGA) 1.2 were used for drawing the profiles across the valley to dis-tinguish the geomorphic units on the basis of their relief. Fig. 2 showsthe geomorphic map of the study area along with major geologicalstructures. Table 2 provides a summary of different geomorphic unitsmapped in this work along with corresponding terminologies used byprevious workers.

Six lithologs were prepared from the exposed sections across differ-ent fan surfaces to describe the stratigraphic framework of fans. To es-tablish the chronology of the fan units, 11 samples were collectedfrom the fan surfaces and terraces in the study area. Samples were col-lected in iron pipes and immediately sealed to prevent the exposure oflight. These sampleswere processed using optically stimulated lumines-cence (OSL) dating technique atWadia Institute of Himalayan Geology,India. The available OSL dates from previous literature (Singh et al.,2001; Thakur et al., 2007; Dutta et al., 2012) were also used, althoughlimited information was available for these samples. Table 3 presentsthe OSL data generated in this work.

4. Major geomorphic units and fan stratigraphy

4.1. Lesser Himalaya hills (LHH)

Lesser Himalaya hills are situated at the hanging wall of the MainBoundary thrust to the north (Fig. 3A). This unit is mainly composed

s workers are represented as F1 and F2. F1 represents MBT—Main Boundary thrust andjahaun fault, BT—Bhauwala thrust, AF—Asan fault, TF—Tons fault; their presence is not—Channel with bars, DN—Drainage network, NTH—Northern thrust sheet, DH—DissectedDistal fan, UGF—Undifferentiated gravel-fill, FP—Floodplain, DS—Dip slope, T—Fill terraces.

Table 2Summary of geomorphic units and their description in Dehra Dun.

Unit Description Comparison withearlier work

Northern thrust sheet(NTH)

Area north of MBT with high drainage density; high relief pattern on DEM, elevation varying from 3000 to 500 m, mainlycomposed of Lesser Himalayan rocks; NDVI values are nearly ‘0’ that indicates barren land.

Mapped as LesserHimalayaa,b

Dissected hills (DH) Highly dissected Siwalik rocks, mainly Middle and upper Siwalik rocks (Sandstone, conglomerate); sparse vegetation at innerdissected hills and thick vegetation cover at outer dissected hills; elevation varies from 1200 to 1000 m, NDVI values (−0.04to 0.60.

Same as suggested byprevious workers

Isolated hills of Siwaliks(IHS)

Low–elevation, rounded residual hills of Upper Siwaliks; elevation ranges from 880 m to 620 m; positive NDVI values (0.040to 0.60); distributary streams, thick vegetation cover.

Pedimented Siwaliks(hills)a,b

Isolated hills of fan (IHF) Low–elevation, rounded residual hills of fan material mainly composed of massive boulders & gravels; elevation varies from840 to 620 m; positive NDVI values (0.040 to 0.60); parallel drainages

Pediments GD-Ia

Piedmont Unit Ab

Proximal Fan (PF) Composed dominantly of gravels and pebble with some boulders; flat surface with less vegetation; altitude varies from 840to 500 m,; NDVI values (−0.04 to +0.04)

Pediments GD-Iaa

Piedmont Unit B2

Distal fan (DF) Youngest gravel unit with thick vegetation cover; elevation ranges from 700 to 480 m; NDVI values (−0.04 and +0.04);slope 0–2°, parallel drainages, dissected surface, Dun gravels,

Pediments GD-III, IVa

Piedmont Unit Cb

Undifferentiated gravel Undifferentiated gravels, identified as fill surface; elevation ranges from 560 to 460 m; few drainages, less vegetation; NDVIvalues (−0.04 to +0.04)

–

Flood plain Active flood plain, fluvial sediments; elevation varies from 660 to 420 m; Gravels, pebbles and coarse sand; NDVI valuesnegative to zero

Floodplainb

Inactive flood plain Abandoned floodplain of Asan river, cultivated areas; elevation varies from 460 to 440 m; thick soil cover; NDVI values nearlyzero.

Floodplainb

Dip slope Dip slope gravels mainly derived from Mohand anticline; elevation varies from 680 to 500 m; slope 5° to 10°; NDVI valueshighly positive (0.40–0.60); thickly vegetated, nearly parallel drainages

Piedmont Unit Da

Terraces Fluvial terraces composed of gravels, pebbles, sand and mud; Flat surfaces, younger sediments; elevation varies from 720 to480 m; NDVI values are nearly zero or slightly positive

–

a Singh et al., 2001.b Thakur et al., 2007.

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of Lesser Himalayan rocks, which include argillites, phyllites, quartzite,carbonate, and slates (Hagen, 1969; Stocklin, 1980; Valdiya, 1980).This area can easily be demarcated on the satellite images because ofits rugged and high relief pattern on the DEM. Slope ranges from 10°to 50° and altitude varies from 500 to 3000m in this unit. The presenceof active lineaments makes this area prone to landslides and debris falls(Fig. 3B) and sediments are transported to the Dun through dendritic tosubdendritic drainage network.

4.2. Dissected hills (DH)

The dissected hills unit is composed of middle and upper Siwalikrocks that are dominantly sandstone and conglomerate, respectively.This unit has been mapped as ‘inner’ and ‘outer’ dissected hills basedon its position in the valley, i.e., north and south of theDun, respectively.The ‘inner dissected hills’ unit is situated on the footwall of theMBT andon the hangingwall of the Santaurgarh thrust (Fig. 3A). The elevation of‘inner dissected hills’ ranges from 1000 to 1200m and slope varies from10° to 30°. The activity of the Santaurgarh thrust ismanifested as deeplyincised valley in the inner dissected hills and presence of large bouldersupstreamof the Koti (Fig. 3C), a very small streamdraining the dun sur-face. The ‘outer dissected hills’ constitute the SW flank of the Mohandanticline. The elevation of the outer dissected hills ranges from 550 to800 m and slope varies from 5° to 30°. This unit is highly dissected byseveral small streams like the Suarna, Koti, Mauti, Tons, and manyminor ephemeral streams that flow parallel to each other.

4.3. Isolated hills (IH)

Isolated hills are low-elevation, rounded residual hills adjacent tothe Himalayan front (inner Siwalik hills, Fig. 3A). The elevation rangesfrom 620 to 880 m and the slope varies from 5° to 20°. On satellite im-ages, this unit can be identified as disjointed hills separated from eachother and at higher elevations compared to the alluvial fans. This unitis covered with thick vegetation (reserved forests) and yields highlypositive NDVI values (0.40–0.60; Table 2). Based on their composition,this unit has been further divided into two subunits, namely isolatedhills of fan material (IHF) and isolated hills of Siwaliks (IHS). The IHS

subunit is mainly composed of sandstones of middle Siwaliks and con-glomerate deposits of upper Siwaliks. This unit is mainly exposed atthe eastern and western margins of the study area (Fig. 2). The con-glomerate deposits are mainly made up of subrounded gravels ofquartzite and mudstone derived from the Lesser Himalayan region.The IHS unit is at a higher altitude in comparison to the IHF andshows a radial drainage pattern.

The IHF subunit is mainly composed of massive boulders, gravels,and pebbles of quartzite, sandstone, and limestonewith a sandymatrix.In general, the IHF sediments represent debris flow deposits with inter-mittent sheet flows as suggested by the dominance of disorganizedgravels and thin layers of matrix-supported organized gravels. Isolatedhills of fan (IHF) unit was identified as GD-I depositional unit by Singhet al. (2001) and depositional unit ‘A’ by Thakur et al. (2007)(Table 2). Three lithologs, IHF1, IHF2, and IHF3, were prepared fromthe exposed sections in this geomorphic unit (Fig. 4A). The IHF1 sectionis only 3 m thick with ~15–20 m of overburden. The matrix-supportedorganized (imbricated) gravels (80 cm thick) are present at the baseof the section, and the sandymatrixwithin thebasal gravel layer yieldedan OSL age of 41.3 ± 1.2 ka. A 20-cm-thick layer of disorganized graveloverlies the base layer that is in turn, overlain by an ~1-m-thick layer ofmatrix-supported organized gravels and finally by a 1-m-thick surfacesoil (Fig. 5b). The IHF2 section at Dharer nala, a small tributary of theTons River, is ~20 m thick and starts with a 50-cm-thick layer of orga-nized gravels at base suggesting channelized flow. This layer is overlainby 2.5m of amassive sand layer that yielded anOSL age of 43.1± 6.8 ka(Table 3, Fig. 4B). The IHF3 section has Siwalik rocks at the base (Fig. 5b),and the IHF deposits are made up of a 2-m-thick layer of disorganizedgravels with some boulders (maximum size ~50 cm). The upper partsof the IHF2 and IHF3 sections have been interpreted as proximal fan de-posits (discussed next).

4.4. Proximal fan (PF)

The proximal fan unit occupies the area between the ‘isolated hills’unit close to the mountain front (Fig. 3D) but extends farther down-stream of the Santaurgarh thrust, ~5–10 km. The preserved fan surfacehas a radius of 4–5 kmand surface area of ~54 km2. The elevation of this

Table 3Age data of fan units and terrace deposits in Dehra Dun based on OSL dating.

Litho-stratigraphicunit

Sample no./Log Depth (m)a U(ppm)

Th(ppm)

K (%) Moist. cont (%) Equivalent dose (De) Gy Dose rate(Gy/ka)

Age(ka)

TerraceSitla Rao

ST-1 1 2.17 ± 0.02 10.7 ± 0.11 2.02 ± 0.02 9.44 2.77 ± 0.48 3.05 ± 0.04 0.9 ± 0.2

TerraceChor Khala

CKT-1 1 2.34 ± 0.02 15.1 ± 0.15 2.45 ± 0.02 21.48 11.70 ± 0.65 3.32 ± 0.07 3.5 ± 0.2

Distal fan FS 2.1 4 2.23 ± 0.02 15.3 ± 0.15 2.91 ± 0.03 3.44 59.68 ± 3.96 4.33 ± 0.05 13.8 ± 0.9Distal fan FS2.2

(DF1)5 2.43 ± 0.02 16 ± 0.16 2.46 ± 0.02 13.53 51.46 ± 1.87 3.58 ± 0.06 14.4 ± 0.6

Distalfan

LIS-TOP 2.8 3.3 ± 0.03 18.1 ± 0.18 3.02 ± 0.03 1.27 80.72 ± 6.82 4.99 ± 0.06 16.2 ± 1.4

Distal fan SU-1 0.9 NA NA NA 12.6 51.25 ± 3.98 2.57 ± 0.18 19.9 ± 2.1Proximal fan FS3.1

(IHF3)10 1.89 ± 0.02 16.4 ± 0.16 2.21 ± 0.02 18.16 65.71 ± 3.62 3.10 ± 0.06 21.2 ± 1.2

Proximal fan FS1.1(PF1)

3 4.56 ± 0.05 21.4 ± 0.21 2.65 ± 0.03 14.38 150.87 ± 14.17 4.47 ± 0.08 33.8 ± 3.2

Proximal fan DHR-2(IHF2)

5 NA NA NA 7.6 111.23 ± 9.84 3.69 ± 0.27 30.1 ± 3.5

Isolated hills IHF1 2.8 3.1 ± 0.03 18.4 ± 0.18 3.08 ± 0.03 1.36 207.20 ± 5.47 5.02 ± 0.06 41.3 ± 1.2Isolated hills DHR-1

(IHF2)9 NA NA NA 4.2 163.73 ± 23.27 3.8 ± 0.28 43.1 ± 6.8

a Sample depth from surface in m.

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unit ranges from 500 to 840m, and slope varies from2° to 5° southward(Table 1). The NDVI values range between−0.04 and +0.04, suggest-ing comparatively less vegetation cover with flat cultivated areas.

The proximal fan unit was identified as depositional unit GD–II(Singh et al., 2001) and unit ‘B’ by Thakur et al. (2007) (Table 2). Thisunit lies unconformably over the steeply-dipping to overturned middleSiwalik sandstones (Thakur et al., 2007) at the hanging wall of theSantaurgarh thrust but shows no evidence of offset or deformation

Fig. 3. Field photographs of different geomorphic units. (A) Overview of geomorphic units fromhills unit covers the area of the footwall of the MBT, and isolated hills unit are mapped as detahanging wall of the MBT is mainly composed of the Lesser Himalayan rocks, which include argis the most dominant mechanism for sediment transport. (C) Field photos of ‘dissected hills’ mthe dissected hills unit indicates tectonic activity of the Santaurgarh thrust. (D) Spatial dunconformably on the middle Siwalik rocks at the hanging wall of the Santaurgarh thrust, and

across the Santaurgarh fault. The exposed thickness of this unit is~50 m. The proximal fan unit is exposed continuously for ~1.5 km up-stream of the Santaurgarh thrust along several major drainages, includ-ing the Koti and the Suarna rivers. At the footwall of the Santaurgarhthrust, this unit appears to overlie the IHF unit (Fig. 3D), and the fanhead has been entrenched by later fluvial processes. The proximal fansediments have clasts ranging from pebbles to gravels and rarelyboulders. Clasts are relatively poorly to fairly sorted but fining upward

north to south. Hanging wall of the MBT is identified as ‘northern thrust sheet’. Dissectedched rounded hills south of the dissected hills. (B) ‘Northern thrust sheet’ mapped in theillites, phyllites, quartzite, carbonate, and slates. Slope failure is common in this unit andainly consist of middle Siwalik rocks. Presence of big boulders at the Koti River outlet inistribution of geomorphic units of IHF and proximal fan. Deposits of the IHF unit lieboth units are unconformably overlain by the proximal fan unit.

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(normal grading), suggesting these to be sheet flowdeposits. These sed-iments were deposited in paleovalleys that were incised by at least70–75 m into the underlying deposits of the IHF. The axes and orienta-tion of these paleo–valleys are slightly offset from those of the moderndrainage system, which means that their width cannot be observed di-rectly (Densmore et al., 2015). The drainages flowing over the fan sur-face are parallel to subparallel. Large boulders were observed in thisunit toward the basin margin (Fig. 5A).

Two lithologs from PF1 and PF2 sections document the stratigraphyof proximal fan sediments. A 6 m high PF1 section is located down-stream of Dharer nala, a small tributary of the Tons River. The proximalfan sediments in this section consist of an organized gravel bed (basenot exposed) overlain by a 3-m-thick disorganized boulder–gravellayer with randomly oriented clasts and loose matrix (Fig. 4B). A thinsand layer above gave an OSL age of 33.8 ± 3.2 ka (Fig. 4B). An 80-cm-thick paleosol layer separates the lower proximal fan sediments inthis section from the upper organized gravel layer, which wasinterpreted to be a part of distal fan deposits (discussed later). ThePF2 section is a reinterpreted section (section D3 of Singh et al., 2001).This section has a massive sand layer (1 m thick) at the base for whichan OSL age of 21.2 ± 1.2 ka was obtained by Singh et al. (2001). Amajor part of this section (10.5 m thick) consists of alternating layersof clast-supported and matrix-supported gravels. A 2-m-thick paleosolabove the sand layer represents a major discontinuity and marks thecessation of proximal fan deposition.

The upper part of the IHF2 section (0–8 m) and the middle part ofthe IHF3 section (6–10 m) have also been interpreted as proximal de-posits onlapping the IHF sediments. In the IHF2 section, the proximalfan deposits consist of a massive sand layer overlain by a 3-m-thicklayer of pebbly sand and 6-m-thick layer ofmatrix-supported organizedgravels. The upper part of this log consists of anothermassive sand layer(4 m thick) that was dated as 30.1 ± 3.5 ka (Table 3, Fig. 4B). The sandlayer is overlain by a 2-m-thick layer of clast-supported organizedgravels, and the section is capped by a 2-m-thick surface soil. In theIHF3 section, the proximal fan deposits are made up of a sand layer(2-m-thick) with a date of 21.2 ± 1.2 ka (Fig. 4B) and a thin layer(1m) of the imbricated gravels, suggesting a brief period of channelizedflow. The overlying paleosol layer (80 cm thick) marks a minor discon-tinuity in the sequence, and the overlying sediments were interpretedto be a part of distal fan deposits (discussed next).

4.5. Distal fan (DF)

The distal fan has been mapped farther south of the proximal fan.The southern stretch of this unit is bounded by the AsanRiver. The distalfan unit is a near-planar surface with elevation varying from 480 to700 m sloping b2° southward. The observed fan radius is ~10–17 km,and the total surface area is ~131 km2. This unit consists of poorlysorted, angular to subangular pebbles with rare boulders (depositionalunit ‘C’ of Thakur et al., 2007). Singh et al. (2001) identified the sameunit as depositional unit GD III and GD IV (Table 2) with pedofacies III.The exposed thickness of this surface varies from ~15m in the northernpart to ~5 m in the southern part. The NDVI values vary from−0.04 to+0.40 for this unit, suggesting barren lands or settlements with someareas of thick vegetation cover. The topography of this unit is highly un-dulating owing to dissection by several minor streams.

A ~10-m-thick exposed section (DF1) on the western bank of theSitala Rao has a 4.5-m-thick layer of disorganized gravels at the baseoverlain by a 1-m-thick sand layer that was dated as 14.4 ± 0.6 ka(Fig. 5B). The sand layer is overlain by a 1.5-m-thick clast-supported or-ganized gravels, and then by a 1-m-thick matrix-supported organizedgravel. In the IHF3 section, the uppermost (0–5.5 m) part of the se-quence has been interpreted as distal fan deposits consisting of twolayers of matrix-supported organized gravels, each ~1 m thick, separat-ed by a thin sandy layer. The section is capped by sandy soil (1 m thick)that is highly disturbed owing to the presence of thick vegetation. Distal

fan deposits are also exposed in the upper part of the PF2 section (0–8 m) consisting of thick alternating layers of clast- and matrix-supported gravels. An intervening sand layer at ~3 m depth in PF2 sec-tion was dated to be ~9.4 ka by Singh et al. (2001).

Three additional OSL dates were obtained for distal fan deposits(Table 3). The sample SU-1 came from an exposed section on the east-ern flank of the Suarna River at a depth of 0.9 m from a location closeto the IHF2 section (Fig. 5A) and gave an OSL date of 19.9 ± 2.1 ka(Table 3). The samples LIS-TOP and FS 2.1 were collected from distalfan deposits overlying the IHF/PF deposits (Fig. 5A) and yielded OSLages of 16.2 ± 1.4 and 13.8 ± 0.9 ka, respectively. Distal fan sedimentsare onlapping those of the proximal fan as observed in several lithologs(IHF3, PF1, PF2) but not all the way up to the hanging wall of theSantaurgarh thrust; this suggests backfilling of this unit over the olderfan units.

4.6. Dip slope (DS)

The dip slope unit has been mapped on the northern flank of theMohand anticline and is mainly controlled by the dip of the SiwalikRanges (mapped as dip-controlled slope by Thakur et al., 2007). Litho-logically, it is mainly composed of unsorted subrounded boulders andpebbles, predominantly of quartzite and sandstone derived from theSiwalik Ranges of the Mohand anticline. The total surface area of thisunit is ~115 km2, mostly covered by thick vegetation covercharacterised by highly positive (0.4–0.6) NDVI values (Table 1). The re-gional slope of this unit varies from 5° to 10°, and the elevation rangesbetween 500 and 680 m. Based on TRMM data, Bookhagen andBurbank (2006) interpreted very high annual rainfall (~100–300 cm)along the Mohand stretch because of a significant topographic high. Asa result, hillslope failures and landslides are common in this unit(Fig. 5B) and they constitute the dominant sediment transport process-es in the area (Barnes et al., 2011). The OSL age of ~30 ka (Thakur et al.,2007) suggests a major unconformity between the underlying Siwalikstrata (age ~0.5 Ma; Sangode et al., 1996) and the dip slope unit.

4.7. Channel belt

Channel belts of the major rivers such as the Koti, Mauti, Sitala Rao,and Suarna occupy the lowest elevations in the Dehra Dun. These sea-sonal rivers dissect the fan surfaces and flow for a length varying from~20 to 25 km from their point of origin in the Lesser Himalaya to theirconfluence with the axial river, the Asan (Tons). The Asan (Tons) origi-nates from southern slopes of theMussoorie hills and drains for ~50 kmbeforemeeting the Yamuna. Channel belts ofmost of the rivers are quitenarrow (b0.5 km) in the upstream reaches, but they becomewider (1 to1.5 km) in downstream reaches. Small streams such as theNimi, Dharer,Ghulata, Udmadi Rao, and Chor khala originatewithin the valley,mainlyfrom the detached residual hills (IH unit). These are ephemeral streamsand most of them are b15 km long. Large boulders were noted in thechannel belt of the Kosi and Mauti rivers in their upstream reaches. Inthe downstream reaches, gravels and pebbles constitute a major partof channel sediments.

4.8. Floodplain (FP)

The major rivers draining the study area, the Yamuna, Suarna, SitalaRao, and Asan, have prominent floodplains. The general slope of thefloodplains ranges from 0° to 1° and the elevation ranges from 420 to660 m. The floodplain unit was demarcated on the satellite images onthe basis of image characteristics such as high moisture content andNDVI values (negative to zero; see Table 1). The floodplain along theAsan River is 0.5–2.5 km wide but is much narrower (600–900 m)along the Suarna River (Fig. 5C). The floodplain unit is composed ofsand and silt and little mud with some pedogenic modification and in-tercalated with pebbles. At the western end of the study area, the

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Fig. 5. Field photos of different geomorphic units. (A) Location of the sample collected from ‘proximal fan unit’. This ~6-m-high section near Dharer nala has an organized gravel bed at thebase that is overlain by disorganized boulder-gravel units. Big boulders were observed in this unit toward the basin margin suggesting high precipitation in catchment. (B) ‘Dip slope’geomorphic unit, mapped on the NW flank of the Mohand anticline, comprised of unconsolidated gravels, unsorted subrounded pebbles, and boulders of quartzite and sandstone.High rainfall makes this unit vulnerable to slope failures. (C) River terraces along the Suarna River. The terraces are ~4–5 km long and ~0.5 km wide and located ~2–3 m above theriver bed. Floodplain width along the river varies from 600 to 900 m. (D) One set of depositional terraces along Chor Khala River. The sample (CKT-1) collected from the right bank ofthe river, ~1 m below the surface, yielded an OSL age of 3.53 ± 0.21 ka.

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Yamuna has a well-developed floodplain, and the width varies from0.5 km in the upstream to 3 km in downstream reaches.

4.9. Terraces (T)

One set of paired fill terraces were mapped along the Sitala Rao andSuarna rivers. Terraces are cultivated areas with elevation ranging from480 to 720 m. Terraces are mainly composed of gravels and pebbles insandy matrix. The NDVI values of these surfaces are nearly zero orslightly positive because of the presence of barren land and cultivatedareas. Terraces along the Sitala Rao are ~5 km long and ~0.5–0.8 kmwide. We obtained an OSL age of 0.9 ± 0.2 ka from a sample takenfrom this terrace (Table 3). Terraces along the Suarna River are ~4 to5 km long and ~0.5 km wide, located ~2 to 3 m above the river bedand are flat with sparse vegetation cover (Fig. 5C). An approximately~30-km-long and 0.7 to 2.5 kmwide asymmetric terrace was identifiedalong the southern bank of the Asan river. This terrace is bordered bythe dip slope (DS) unit at the south and is cut by several ephemeral

Fig. 4. (A) Locations of various lithologs andOSL samples on the satellite image. (B) Lithologs andIHF deposits that are interpreted as debris flow deposits withmaximumOSL ages of ~43/41 ka.fan deposits respectively, yielding younger ages. Lithologs PF1 and PF2 document proximal fanupper parts. Litholog DF1 is exposed at the west bank of Sitala Rao and has 4.5-m-thick unit ofdeposits.

streams flowing from the northern flank of the Mohand anticline thatjoin the Asan River to the south. The formation of this asymmetric ter-race along the Asanmay be related to the uplift of theMohand anticline.Another small symmetric depositional terrace surface, ~0.5 km long,~0.2 km wide, and ~1 m high, was identified along the Chor KhalaRiver (Fig. 5D), a small tributary of the Asan River. An OSL date of3.5 ± 0.2 ka was obtained for a sample from this terrace (Table 3).

4.10. Undifferentiated gravels (UGF)

This unit, composed mainly of gravels, is flat and occupies a totalarea of ~200 km2 north of the Asan river and east of the Tons river. Inthe surface profile, there is no major break between distal fan unit andundifferentiated gravels unit. A large part of this unit is dotted with cul-tivated land and settlements including the city of Dehradun. Although ithas beenmapped as a fairly continuous unit, it is quite scattered and dis-jointed owing to human interventions.

chronologyof all sections exposing different fan units. IHF1, IHF2, and IHF3 logs documentThe upper parts of the IHF2 and IHF3 sections have been interpreted as proximal and distaldeposits with ages of 33–21 ka. The PF1 and PF2 sections record distal fan deposits in thedisorganized gravels. Thin (~1 m) sandy layer yields OSL age of 14.4 ± 0.6 ka for distal fan

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5. Discussion

5.1. Tectonogeomorphic evolution and depositional history of the DehraDun

The IHF unit is a steeply dipping fan surface and is mainly composedof clast- and matrix-supported boulders to pebble conglomerate. Theclasts are poorly sorted and randomly oriented suggesting dominantdebris flow deposits (Blair and McPherson, 1994; Singh et al., 2001;Nichols, 2009). The deposition of isolated hills of fan IHF unit occurredduring ~43–33 ka coinciding with the precipitation minima around40 ka (Prell and Kutzbach, 1987). We argue that the availability of sed-iments in the system was related to reactivation of MBT during70–55 ka (Banerjee et al., 1999) and 50–35 ka (Singhvi et al., 1994;Singh et al., 2001) that led to the deposition of thick strata of IHF(≥41 ka) in the Dun (Fig. 6). Low transport capacity caused by the pre-cipitation minimum during this period resulted in deposition close tothe mountain front.

The proximal fan unit primarily consists of matrix- and clast-supported pebbles to gravels with some boulders having loose contactwith matrix that suggests mud flow or sheet flow deposits (Blair andMcPherson, 1994; Singh et al., 2001)withminor debris flows. The prox-imal fan unit is the second phase (34–21 ka) ofmajor aggradation in theDun that coincides with a period of high sediment production from theHimalaya (Sharma and Owen, 1996; Taylor and Mitchell, 2000; Pantet al., 2006). This is also the period of high precipitation that led tohigh intensity floods in the foreland induced by high solar insolationat ~31 ka (Goodbred, 2003). High sediment supply (Lecce, 1990)and an increase in water-sediment ratio (Ballance, 1984; Wells andHarvey, 1987; Harvey and Wells, 2003; Densmore et al., 2007;Nichols, 2009) during this period were the primary reasons for transi-tion from debris flow deposits (IHF unit) to sheet flow deposits (PFunit). The elevation difference of ~100 m between the IHF and PFunits (Table 2) suggests an incision of the IHF unit prior to the deposi-tion of proximal fan unit.

The onlapping of the proximal fan deposits over the IHF deposits(lithologs IHF2 and IHF3) at the hanging wall of the Santaurgarh thrustsuggests widespread deposition of the proximal fan unit during theLGM and eventually backfilling toward and across the Santaurgarh

Fig. 6. Thematic section across the Mohand anticline and the Lesser Himalaya. Major geomorpproximal fan (PF), and distal fan (DF) are the three major fan surfaces with the ages N41, 34–and isolated hills unit is mapped as detached rounded hills south of the dissected hills. Dip slo

thrust. This backfilling of the proximal fan deposits could have beentriggered because of tectonic influences (e.g., increase in slip rate; Bull,1961, 1964; DeCelles et al., 1991; Gordon and Heller, 1993; Mellere,1993; Allen and Densmore, 2000). The increase in slip rate provides ac-commodation space and the younger fan unit can onlap the older units(Densmore et al., 2007). Backfilling can also be influenced by hydraulicconditions such as less water availability or low water-sediment ratiothat can lead to the onlapping of the younger unit over the older oneclose to the mountain front (Goudie, 2004; Densmore et al., 2007;Hamilton et al., 2013). However, as no deformation was observed onthe fan surfaces in the Dun and the widespread filling occurred aroundthe LGM time, we argue that climatic factors have had a dominant rolein back filling of the proximal fan unit.

The presence of boulders in the distal part of the fan unit (Fig. 4A)has been debated and alternative hypotheses have been proposed.Kumar et al. (2007) and Giano (2011) interpreted the presence of boul-ders in fan deposits toward the basin margin because of high precipita-tion in the catchment. On the contrary, Singh et al. (2001) suggestedthat the development of the pediments 1 and 2 (IHF in the presentwork) caused by the activity of the Bhauwala thrust and headward ero-sion of the streams. They also suggested that the pedimented Siwalik(IHS in the present work) and fan area were uplifted caused by the ac-tivity of the Bansiwala thrust (Asan fault according to Thakur et al.,2007). In the hanging wall of the Santaurgarh thrust, fan depositswere observed lying unconformably over the steeply dipping tooverturned middle Siwalik sandstones (Thakur et al., 2007), but no ev-idence was recorded for any deformation in the fan units across theSantaurgarh thrust. We therefore favour the climatic interpretation(high precipitation) for such deposits.

The distal fan unit consisting of poorly sorted, angular to subangularpebbles have been interpreted as braided streamdeposits and overbankdeposits. The distal fan unit (20–11 ka) represents the third phase of de-position in the Dun. Sheet-like fan deposits indicate high precipitationin the catchment area (Densmore et al., 2007). The timing of distal fandeposition coincides with the post-LGM period that corresponds tohigh sediment supply from the hinterland because of monsoonal inten-sification (Overpeck et al., 1996) and to extensive slope failures and hillslope sediment transport (Pratt et al., 2002). The lithologs PF1 and PF2clearly show that the distal fan sediments onlapped the proximal fan

hic units and structures in the Dun are marked on the section. Isolated hills of fan (IHF),21 and 20–11 ka, respectively. Dissected hills unit is mapped at the footwall of the MBTpe geomorphic unit was mapped on the NW flank of the Mohand anticline.

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deposits. This again suggests backfilling of distal fan deposits (Fig. 6) butnot up to thehangingwall of the Santaurgarh thrust.We argue that suchbackfilling is related to water-sediment ratio influencing the transportcapacity of the river systems (Goudie, 2004; Densmore et al., 2007;Hamilton et al., 2013).

During the early to mid-Holocene period, minor terrace depositsdating 11 ka (Singh et al., 2001) and 3 ka (presentwork) have been doc-umented along the rivers in the Dun. This period was the beginning ofthe incision phase in the Dun because of intense monsoonal activity(Singh et al., 2001). An incision phase started in the Duns at ~12/10 kaand has continued until present (Singh et al., 2001; Suresh andKumar, 2009). No depositionwas recorded after 3 ka except aminor re-cent aggradational terrace (OSL age 0.9 ka, sample ST-1) along the SitalaRao (Fig. 4A).

5.2. Conceptual model

Fig. 7 shows the sequential development of the different fan units inthe Dehra Dun. The deposition of the IHF unit (Stage I) began at ~43 kaand probably ended around ~33 ka. The reactivation of MBT during75–65 and 50–35 ka (Singhvi et al., 1994) provided high sediment avail-ability and low precipitation minima (Prell and Kutzbach, 1987),resulting in low transport capacity and deposition of sediments closeto the mountain front. The geomorphic expression of this fan is notwell preserved because of several cycles of incision. Therefore, the

Fig. 7. Evolutionarymodel of the Dehra Dun fills. (I) IHF deposits formed during 43–33 ka, and thlow precipitation minima, resulting in low transportation capacity that resulted in sedimentdeposition from 34 to 21 ka and developed between isolated hills; field evidences suggest inc(Singh et al., 2001) provided high sediment availability during this period and led to continuolocus shifted southward owing to fanhead entrenchment of the proximal fan. (IV) A domina3–1 ka. (DD—Dehra Dun; PD—Pinjaur Dun, KD—Kiarda Dun, PaD—Parduni basin, CD—Chitwan

precise timing of deposition and abandonment of this unit could notbe ascertained. The proximal fan formed because of continuous deposi-tion from 34 to 21 ka and developed between isolated hills. Field evi-dences suggest incision of isolated hills and refilling of the valley. Theactivity of the Santaurgarh thrust before 23 ka (Singh et al., 2001) pro-vided high sediment availability during this period and led to continu-ous deposition of PF until 21 ka. The distal fan (stage III) formedowing to further southward shift of depositional locus after the aban-donment of the proximal fan and fanhead entrenchment. Depositionof the distal fan sediments (~20–11 ka) may have progressed asbackfilling over the proximal fan deposits (34–21 ka), but no distal fandeposits are observed over the hanging wall of the Santaurgarh thrust.A minor depositional phase is also documented in the intermontanebasin during 3–1 ka (stage IV) that led to thin depositional terracesalong the modern rivers.

All four depositional units, namely isolated hills of fan, proximalfan, distal fan, and younger depositional terraces, are separated bystrong incision phases (Fig. 7). The timing and duration of the inci-sion phases are not precisely constrained, but these must have oc-curred after the deposition of fan units, at ~33 ka for IHF, ~20 kafor PF, and ~10 ka for DF. Earlier work (Densmore et al., 2015) esti-mated that these incision episodes contributed a significant amountof sediment discharge into the plains that were ca. 1–2% of thepresent-day suspended load of the Ganga and Yamuna rivers thattraverse the margin of the Dun.

e reactivation ofMBT during 75–65 and 50–35 ka provided high sediment availability ands deposited close to the mountain front. (II) Proximal fan formed because of continuousision of IH and refilling of the valley. The activity of the Santaurgarh thrust before 23 kaus deposition of PF until 21 ka. (III) Distal fan (20–11 ka) formed when the depositionalnt incision phase (b5 ka) in the Dun with deposition of minor younger terraces duringDun).

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5.3. Comparison with other duns (intermontane valleys) and regionalcorrelation

TheOSL ages suggest that the fan development inwestern Dehradunstarted at ~43 ka and ceased around 10 ka. Aggradational phase I (43–33 ka) that corresponds to formation of the IHF unit is comparable tothe first aggradation phase in Yamuna valley (~37–24 ka) betweenthe MBT and the HFT (Dutta et al., 2012). An aggradational phase wasalso documented during ~49–25 ka at various upstream sites in theGanga basin (Srivastava et al., 2008; Ray and Srivastava, 2010). Fan ag-gradation in the Pinjaur Dun was also documented from 72 to 24 ka(Kumar et al., 2007; Suresh et al., 2007) but without any incisionphase as recorded in Dehra Dun (Fig. 7).

Phase II (~34–21 ka) of proximal fan deposition correlates with de-positional phases in the Soan Dun in NW Himalaya during ~36–29 ka(Suresh and Kumar 2009) and falls within the fan aggradation phase(~72–24 ka) in the Pinjaur Dun in NW Himalaya (Kumar et al., 2007;Suresh et al., 2007). Fan aggradation phases in the Kiarda and theParduni basins in NW Himalaya –recorded from ~34 to 19 ka andfrom ~27 to 20 ka, respectively (Philip et al., 2009) – are also correlat-able to this phase (Fig. 7). In Chitwan Dun in the Narayani basin, south-ern Nepal, no absolute ages of the fill deposits are available, but Kimura(1995) suggested ages of the depositional units as ~26–16 and b10 kaseparated by an incision event.

Aggradational phase III (20–11 ka) represented by distal fan deposi-tion in Dehra Dun, correlates with the second aggradation phase(15–12 ka) in the Yamuna valley between the MBT and the HFT(Dutta et al., 2012). Aggradation in the Ganga valley between MBTand HFT was also documented at ~11 ka followed by incision during11–9.7 ka (Sinha et al., 2010). A phase of aggradation was documentedat several upstreamsites in the Ganga basin during 18–11 ka (Srivastavaet al., 2008; Ray and Srivastava, 2010). A minor depositional phase wasalso observed in the Pinjaur Dun during 16 ka (Suresh et al., 2007).Minor terrace deposits during 3–1 ka in the Dehra Dun (aggradationalphase IV) correlates with a brief depositional event in the Yamuna val-ley during (~5–1 ka) (Dutta et al., 2012) and the Pinjaur Dun during4.5 ka (Fig. 7) (Suresh et al., 2007). Fan aggradation ceased ~11 ka agoin the Dehra Dun (Singh et al., 2001) and so did the second phase offan deposition in the Kangra region around 10 ka (Suresh and Kumar,2009).

It has been observed that major fan aggradational and incisionphases in the Duns along the Himalayan arc (Kumar et al., 2007;Suresh et al., 2007; Philip et al., 2009; Suresh and Kumar, 2009; Sinhaet al., 2010; Dutta et al., 2012) correlate reasonablywell, but spatial var-iation in timing of individual events is noted. We therefore suggest thatthese Duns have gone through similar cycles of sedimentation/evacua-tion under the influence of climatic variation with interruptions fromtectonic activity and sediment supply from the mountainous area.Major aggradational phases are related to the Quaternary glacial period,whereas the degradational phases are related to interglacial periods(Harvey, 1990). We suggest that the role of tectonic activities hasbeen limited to creation of accommodation space in the Dehra Dun re-gion and has not influenced the fan development process in a majorway.

Our observations are in linewith landscapedevelopment in differentparts of the Himalaya documented on the basis of sedimentological,geomorphic and chronostratigraphic analysis using OSL and terrestrialcosmogenic nuclide (TCN). These studies have suggested that erosionaland depositional processes throughout the Himalaya and Tibet havebeen mainly controlled by climatic changes, specifically the transitionbetween glacial to deglacial periods (Barnard et al., 2004, 2006a,2006b; Dortch et al., 2009; Seong et al., 2009; Ray and Srivastava,2010). In the Central Karakoram region, glacier geochronology and out-burst flood deposits have indicated that landform development wasdominantly influenced by glaciation to paraglaciation during the lateQuaternary (Seong et al., 2009). Paleoclimatic records from the

Alaknanda–Ganga system in the Lesser Himalaya indicate that aggrada-tion and incision phases between 63 and 11 ka resulted primarily be-cause of glaciation–deglaciation processes (Ray and Srivastava, 2010).Mass wasting (landslides) and sheet flow processes during the transi-tion between glacial and nonglacial periods (Barnard et al., 2006a,2006b; Dortch et al., 2009; Seong et al., 2009) also contributed to thesediment input in the Duns (Chandel and Singh, 2000; Singh et al.,2001). However, it has been suggested that tectonic activities along lon-gitudinal as well as transverse faults have continuously modified thelandforms in the Duns (Kimura, 1994, 1999; Chandel and Singh, 2000;Singh et al., 2001; Virdi et al., 2006; Philip and Virdi, 2007; Singh andTandon, 2008; Philip et al., 2009).

6. Conclusions

Geomorphic and stratigraphic investigations in the Dehra Dun inNWHimalaya have suggested that at least three phases of fan sedimen-tation occurred during the late Quaternary period (I) 43–33 ka, (II) 34–21 ka, and (III) 20–11 ka followed byminor terrace formation during 3–1 ka. Each depositional phase was followed by fanhead incision, aban-donment of the active depositional locus and shifting of the depocentertoward the basin margin. Climate change (high precipitation) was theprimary reason for the transition from debris flow deposits to sheetflow and stream flow deposits in the Dun. While the evolutionaryhistory of the alluvial fans in the Dun was influenced by tectonicactivities, particularly in terms of providing accommodation space, theaggradation and entrenchment of fans were controlled by climaticperturbations.

Acknowledgements

This work was supported by a research grant from the Departmentof Science and Technology, Government of India (SR/S4/ES-415/2009)including the studentship to SS and we thankfully acknowledge thissupport. A part of the field investigations and OSL dating were support-ed from a grant from UKIERI (IND/CONT/07-08/199), British Councilthat was crucial for this research. Discussions with AlexanderDensmore, Jason Barnes, Sampat Tandon, Malay Mukul, Vikrant Jain,and Vimal Singh during field work and project review meetings weremost useful to develop the ideas presented in this paper. We thank allthe reviewers for their detailed comments that helped to improve themanuscript significantly. We are grateful to Richard Marston for hispainstaking editorial comments.

References

Allen, P.A., Densmore, A.L., 2000. Sediment flux from an uplifting fault block. Basin Res. 12,367–380.

Ballance, P.F., 1984. Sheet-flow-dominated gravel fans of the non-marine middle Cenozo-ic simmler formation, central California. Sediment. Geol. 38, 337–359.

Banerjee, D., Singhvi, A.K., Pande, K., Gogte, V.D., Chandra, B.P., 1999. Towards a direct dat-ing of fault gouges using luminescence dating techniques—methodological aspects.Curr. Sci. 77, 256–268.

Barnard, P.L., Owen, L.A., Sharma, M.C., Finkel, R.C., 2004. Late Quaternary (Holocene)landscape evolution of a monsoon influenced high Himalayan valley, Gori Ganga,Nanda Devi, NE Garhwal. Geomorphology 61, 91–110.

Barnard, P.L., Owen, L.A., Finkel, R.C., Asahi, K., 2006a. Landscape response to deglaciationin a high relief, monsoon-influenced alpine environment, Langtang Himalaya, Nepal.Quat. Sci. Rev. 25, 2162–2176.

Barnard, P.L., Owen, L.A., Finkel, R.C., 2006b. Quaternary fans and terraces in the KhumbuHimalaya, south of Mt. Everest: their characteristics, age and formation. J. Geol. Soc.Lond. 163, 383–400.

Barnes, J.B., Densmore, A.L., Mukul, M., Sinha, R., Jain, V., Tandon, S.K., 2011. Interplay be-tween faulting and base level in the development of Himalayan frontal fold topography.J. Geophys. Res. Earth Surf. 116, F03012. http://dx.doi.org/10.1029/2010JF001841.

Blair, T.C., McPherson, J.G., 1994. Alluvial fans and their natural distinction from riversbased on morphology, hydraulic processes, sedimentary processes and facies assem-blages. J. Sediment. Res. A64, 450–489.

Bookhagen, B., Burbank, D.W., 2006. Topography, relief, and TRMM-derived rainfall vari-ations along the Himalaya. Geophys. Res. Lett. 33, 1–5.

Bull, W.B., 1961. Tectonic significance of radial profiles of alluvial fans in western FresnoCounty, California. United States Geological Survey Professional Paper 424-B, 182–184.

31S. Sinha, R. Sinha / Geomorphology 266 (2016) 20–32

Bull, W.B., 1964. Geomorphology of segmented alluvial fans in western Fresno County,California: U.S. Geological Survey Professional Paper 352-E, 89–129.

Chandel, R.S., Singh, I.B., 2000. Morphostratigraphy and Fan Morphology of Doon Valley.J. Indian Soc. Remote Sens. 28 (4), 265–275.

DeCelles, P.G., Gray, M.B., Ridgway, K.D., Cole, R.B., Pivnik, D.A., Pequera, N., Srivastava, P.,1991. Controls on synorogenic alluvial-fan architecture, Beartooth Conglomerate(Palaeocene),Wyoming and Montana. Sedimentology 38, 567–590.

Densmore, A.L., Allen, P.A., Simpson, G., 2007. Development and response of a coupledcatchment fan system under changing tectonic and climatic forcing. J. Geophys. Res.112, F01002. http://dx.doi.org/10.1029/2006JF000474.

Densmore, A.L., Sinha, R., Sinha, S., Tandon, S.K., Jain, V., 2015. Sediment storage and re-lease from Himalayan piggyback basins and implications for downstream river mor-phology and evolution. Basin Res. http://dx.doi.org/10.1111/bre.12116.

Dortch, J., Owen, L.A., Haneberg, W.C., Caffee, M.W., Dietsch, C., Kamp, U., 2009. Natureand timing of mega-landslides in northern India. Quat. Sci. Rev. 28, 1037–1056.

Dühnforth, M., Densmore, A.L., Ivy-Ochs, S., Allen, P.A., Kubik, P.W., 2007. Timing and pat-terns of debris flow deposition on Shepherd and Symmes creek fans, Owens Valley,California, deduced from cosmogenic 10Be. J. Geophys. Res. 112, F03S15. http://dx.doi.org/10.1029/2006JF000562.

Dutta, S., Suresh, N., Kumar, R., 2012. Climatically controlled late Quaternary terrace stair-case development in the fold and -thrust belt of the Sub Himalaya. Palaeogeogr.Palaeoclimatol. Palaeoecol. 356-357, 16–26.

Giano, S.I., 2011. Quaternary alluvial fan systems of the Agri intermontane basin (south-ern Italy): tectonic and climatic controls. Geol. Carpath. 62 (1), 65–76.

Goodbred, S.L., 2003. Response of the Ganges dispersal system to climate change: asource-to-sink view since the last interstade. Sediment. Geol. 162, 83–104.

Gordon, I., Heller, P.L., 1993. Evaluating major controls on basinal stratigraphy, Pine Val-ley, Nevada: implications for syntectonic deposition. Geol. Soc. Am. Bull. 105, 47–55.

Goswami, P.K., Pant, C.C., 2007. Geomorphology and tectonics of Kota-Pawalgarh Duns,Central Kumaun Sub-Himalaya. Curr. Sci. 92 (5), 685–690.

Goudie, A.S., 2004. Encyclopedia of Geomorphology vol. 2. Psychology Press (1156 pp.).Hagen, T., 1969. Report on the Geological Survey of Nepal. Preliminary Reconnaissance.

Denkschriften Der Schweizerischen Naturforschenden Gesellschaft 86 vol. 1, p. 185.Hamilton, P.B., Strom, K., Hoyal, D.C.J.D., 2013. Autogenic incision-backfilling cycles and

lobe formation during the growth of alluvial fans with supercritical distributaries.Sedimentology 60, 1498–1525.

Harvey, A.M., 1990. Factors influencing Quaternary alluvial fan development in southeastSpain. In: Rachocki, A.H., Church, M. (Eds.), Alluvial Fans: A Field Approach. Wiley,New York, pp. 247–269.

Harvey, A.M., 2004. The response of dry-region alluvial fans to late Quaternary climaticchange. In: Alsharhan, A.S., Wood, W.W., Goudie, A.S., Fowler, A., Abdellatie, E.M.(Eds.), Desertification in the Third Millenium. Balkema, Rotterdam, pp. 83–98.

Harvey, A.M., Wells, S.G., 2003. Late Quaternary variations in alluvial fan sedimentologicand geomorphic processes, Soda Lake Basin, eastern Mojave Desert, California. In:Enzel, Y., Well, S.G., Lancaster, N. (Eds.), Paleoenvironments and Paleohydrology ofthe Mojave and Southern Great Basins Deserts. Geological Society of America SpecialPaper Vol. 368, pp. 207–230.

Harvey, A.M., Silva, P.G., Mather, A.E., Goy, J.L., Stokes, M., Zazo, C., 1999a. The impact ofQuaternary sea level and climatic change on coastal alluvial fans in the Cabo deGata Ranges, southeast Spain. Geomorphology 28, 1–22.

Harvey, A.M., Wigand, P.E., Wells, S.G., 1999b. Response of alluvial-fan systems to the LatePleistocene to Holocene climatic transition-contrast between the margins of pluviallakes Lahontan and Mojave, NV and CA, USA. Catena 36, 255–281.

Karunakaran, C., Ranga Rao, A.R., 1979. Status of exploration for hydrocarbon in the Hima-layan region—contributions to stratigraphy and structure. Miscellaneous Publicationsof the Geological Survey of India Vol. 41, pp. 1–66.

Kimura, K., 1994. Formation and deformation of river terraces in the Hetauda Dun, CentralNepal. A contribution to the study of post Siwalik tectonics. Sci. Rep. Tohoku Univ. 44(2), 151–181.

Kimura, K., 1995. Terraced debris and alluvium as indicators of the Quaternary structuraldevelopment of the Northwestern Chitwan Dun, Central Nepal. Sci. Rep. TohokuUniv. 45 (2), 103–120.

Kimura, K., 1999. Diachronous evolution of sub-Himalayan piggyback basins, Nepal. Is-land Arc 8, 99–113.

Kirby, E., Whipple, K., 2001. Quantifying differential rock-uplift rates via stream profileanalysis. Geology 29 (5), 415–418.

Kumar, R., Suresh, N., Sangode, S.J., Kumaravel, V., 2007. Evolution of the Quaternary allu-vial fan system in the Himalayan foreland basin: implications for tectonic and climat-ic decoupling. Quat. Int. 159, 6–20.

Lavé, J., Avouac, J.P., 2000. Active folding of fluvial terraces across the Siwalik Hills,Himalayas of central Nepal. J. Geophys. Res. Solid Earth 105 (B3), 5735–5770.

Lavé, J., Avouac, J.P., 2001. Fluvial incision and tectonic uplift across the Himalayas of cen-tral Nepal. J. Geophys. Res. 106, 26561–26591.

Lecce, S.A., 1990. The alluvial fan problem. In: Rachocki, A.H., Church, M. (Eds.), AlluvialFans: A Field Approach. Wiley, Chichester, pp. 3–24.

Malik, J.N., Shah, A.A., Sahoo, A.K., Puhan, B., Banerjee, C., Shinde, D.P., Juyal, N., Singhvi,A.K., Rath, S.K., 2010. Active fault, fault growth and segment linkage along the Janaurianticline (frontal foreland fold), NW Himalaya, India. Tectonophysics 483 (3–4),327–343.

Mather, A.E., Stokes, M., 2003. Long-tern1 landform evolution in southern Spain. Geomor-phology 50, 151–171.

Mather, A.E., Martin, J.M., Harvey, A.M., Braga, J.C., 2009. A Field Guide to the NeogeneSedimentary Basins of the Almeria Province. John Wiley and Sons, South-east Spain(368 pp.).

Mellere, D., 1993. Thrust-generated, back-fill stacking of alluvial fan sequences, south-central Pyrenees, Spain (La Pobla de Segur Conglomerates). In: Tectonic Controls

and Signatures in Sedimentary Successions (ed. by Frostick L.E., Steel R.J.). Int.Assoc. Sedimentol. Spec. Publ. 20, 259–276.

Nakata, T., 1972. Geomorphic history and crustal movements of the foothills of theHimalayas. Sci. Rep. Tohoku Univ. 22, 39–177.

Nakata, T., 1975. On Quaternary Tectonics around the Himalayas. 7th Series (Geography)Vol. 25. Report of Tohoku University, Japan, pp. 111–118.

Nichols, G., 2009. Sedimentology and Stratigraphy. second ed. Wiley-Blackwell (432 pp.).Nossin, J.J., 1971. Outline of the geomorphology of the Doon valley, northern U.P., India.

Zeitschrift Geomorphology N.F. 12, 18–50.Ori, G.G., Friend, F.P., 1984. Sedimentary basins formed and carried piggyback on active

thrust sheets. Geology 12, 475–478.Overpeck, J., Anderson, D., Trumbore, S., Prell, W., 1996. The southwest Indian monsoon

over the last 18000 years. Clim. Dyn. 12, 213–225.Pant, P., Hegde, P., Dumka, U.C., Sagar, R., Satheesh, S.K., Moorthy, K.K., Saha, A., Srivastava,

M.K., 2006. Aerosol characteristics at a high-altitude location in central Himalayas:optical properties and radiative forcing. J. Geophys. Res. 111, 1–9.

Philip, G., Virdi, N.S., 2007. Active faults and neotectonic activity in the Pinjaur Dun, north-western Frontal Himalaya. Curr. Sci. 92 (4), 532–542.

Philip, G., Virdi, N.S., Suresh, N., 2009. Morphotectonic evolution of Parduni Basin: anintradun piggyback basin in Western Doon Valley, NW Outer Himalaya. J. Geol. Soc.India 74, 189–199.

Pratt, B., Burbank, D.W., Heimsath, A., Ojha, T., 2002. Impulsive alluviation during earlyHolocene strengthened monsoons, central Nepal Himalaya. Geology 30, 911–914.

Prell, W.L., Kutzbach, J.E., 1987. Monsoon variability over the past 150,000 years.J. Geophys. Res. 92 (D7), 8411–8425.

Raiverman, V., Kunte, S.V., Mukherjee, A., 1983. Basin geometry, Cenozoic sedimentationand hydrocarbon prospects in north-western Himalaya and Indo-Gangetic plains.Petrol. Asia J. 6, 67–92.

Raiverman, V., Srivastava, A.K., Prasad, D.N., 1994. Structural style in northwestern Hima-layan foothills. Himal. Geol. 15, 263–280.

Rao, D.P., 1978. Geomorphology and Neotectonics: An Example from Sub Himalaya. Proc.Sym. OnMorphology and Evolution of Landforms. Geology Department, University ofDelhi, pp. 88–98.

Ray, Y., Srivastava, P., 2010. Widespread aggradation in the mountainous catchment ofthe Alaknanda-Ganga River System: timescales and implications to hinterland-foreland relationships. Quat. Sci. Rev. 29, 2238–2260.

Ritter, J.B., Miller, J.R., Enzel, Y., Wells, S.G., 1995. Reconciling the roles of tectonism andclimate in Quaternary alluvial fan evolution. Geology 23, 245–248.

Robustelli, G., Luca, L., Corbi, F., Pelle, T., Dramis, F., Fubelli, G., Scarciglia, F., Muto, F.,Cugliari, D., 2009. Alluvial terraces on the Ionian coast of northern Calabria,southern Italy: implications for tectonic and sea level controls. Geomorphology106, 165–179.

Sangode, S.J., Kumar, R., Ghosh, S.K., 1996. Magnetic polarity of the Siwalik sequence ofHaripur area (HP), NW Himalaya. J. Geol. Soc. India 47, 683–704.

Seeber, L., Gornitz, V., 1983. River profiles along the Himalayan arc as indicators of activetectonics. Tectonophysics 92 (4), 335–367.

Seong, Y.B., Bishop, M.P., Bush, A., Clendon, P., Copland, P., Finkel, R., Kamp, U., Owen, L.A.,Shroder, J.F., 2009. Landforms and landscape evolution in the Skardu, Shigar andBraldu Valleys, Central Karakoram Mountains. Geomorphology 103, 251–267.

Sharma, M.C., Owen, L.A., 1996. Quaternary glacial history of the Garhwal Himalaya, India.Quat. Sci. Rev. 15, 335–365.

Silva, P.G., Harvey, A.M., Zazo, C., Goy, J.L., 1992. Geomorphology, depositional style andmorphometric relationships of Quaternary alluvial fans in the Guadalentin Depres-sion (Murcia, Southeast Spain). Z. Geomorphologie, N.F. 36, 325–341.

Singh, A.K., 1998. Geomorphology, Sedimentology and Pedology of the IntermontaneDehradun Valley, Northwest Himalaya (Unpublished PhD. thesis) University ofRoorkee (247 pp.).

Singh, A.K., Prakash, B., Mohindra, R., Thomas, J.V., Singhavi, A.K., 2001. Quaternary alluvi-al fan sedimentation in the Dehradun Valley Piggyback Basin, NWHimalaya: tectonicand palaeoclimatic implications. Basin Res. 13, 449–471.

Singh, V., Tandon, S.K., 2007. Evidence and consequences of tilting of two alluvial fans inthe Pinjaur Dun, Northwestern Himalayan Foothills. Quat. Int. 159, 21–31.

Singh, V., Tandon, S.K., 2008. The Pinjaur Dun (intermontane longitudinal valley) and as-sociated active mountain fronts, NW Himalaya: tectonic geomorphology andmorphotectonic evolution. Geomorphology 102, 376–394.

Singh, V., Tandon, S.K., 2010. Integrated analysis of structures and landforms of an inter-montane longitudinal valley (Pinjaur Dun) and its associated mountain fronts in theNW Himalaya. Geomorphology 114, 573–589.

Singhvi, A.K., Banerjee, D., Pande, K., Gogte, V., Valdiya, K.S., 1994. Luminescence studieson neotectonic events in south-central Kumaun Himalaya—a feasibility study. Quat.Sci. Rev. 13 (5–7), 595–600.

Sinha, S., Suresh, N., Kumar, R., Dutta, S., Arora, B.R., 2010. Sedimentologic and geomor-phic studies on the Quaternary alluvial fan and terrace deposits along the Gangaexit. Quat. Int. 227, 87–113.

Srivastava, P., Tripathi, J.K., Islam, R., Jaiswal, M.K., 2008. Fashion and phases of late Pleis-tocene aggradation and incision in the Alaknanda River Valley, western Himalaya,India. Quat. Res. 70, 68–80.

Suresh, N., Kumar, R., 2009. Variable period of aggradation and termination history of twolate Quaternary aluvial fan in the Soan Dun, NW Sub Himalaya: impact of tectonicand climate. Himal. Geol. 30, 155–165.

Suresh, N., Bagati, T.N., Thakur, V.C., Kumar, R., Sangode, S.J., 2002. Optically stimulated lu-minescence dating of alluvial fan deposits of Pinjaur Dun, NW Sub Himalaya. Curr. Sci.82 (10), 1267–1272.

Suresh, N., Bagati, T.N., Kumar, R., Thakur, V.C., 2007. Evolution of Quaternary alluvial fansand terraces in the intermontane Pinjaur Dun, Sub-Himalaya, NW India: interactionbetween tectonics and climate change. Sedimentology 54, 809–833.

32 S. Sinha, R. Sinha / Geomorphology 266 (2016) 20–32

Stöcklin, J., 1980. Geology of Nepal and its regional frame. J. Geol. Soc. Lond. 137, 1–34.Taylor, P.J., Mitchell, W.A., 2000. The Quaternary glacial history of the Zanskar range,

north-west Indian Himalaya. Quat. Int. 65-66, 81–99.Thakur, V.C., 1995. Geology of Dun Valley, Garhwal Himalaya: neotectonics and coeval de-

position with fault-propagation folds. Himal. Geol. 6 (2), 1–8.Thakur, V.C., Pandey, A.K., 2004. Late Quaternary tectonic evolution of dun in fault bend

propagated fold system, Garhwal Sub-Himalaya. Curr. Sci. 87 (11), 1567–1576.Thakur, V.C., Pandey, A.K., Suresh, N., 2007. Late Quaternary–Holocene evolution of dun

structure and the Himalayan Frontal Fault zone of the Garhwal Sub-Himalaya, NWIndia. J. Asian Earth Sci. 29, 305–319.

Valdiya, K.S., 1980. Geology of Kumaun Lesser Himalaya. Wadia Institute of HimalayanGeology, Dehra Dun, India, p. 291.

Virdi, N.S., Philip, G., Bhattacharya, S., 2006. Neotectonic activity in theMarkanda and BataRiver basins, Himachal Pradesh, NW Himalaya: a morphotectonic approach. Int.J. Remote Sens. 27 (10), 2093–2099.

Viseras, C., Calvache, M.L., Soria, J.M., Fernandez, J., 2003. Differential features of alluvialfans controlled by tectonic or eustatic accommodation space. Examples from theBetic Cordillera, Spain. Geomorphology 50, 181–202.

Wells, S.G., Harvey, A.M., 1987. Sedimentologic and geomorphic variations in stormgenerated alluvial fans, Howgill Fells, northwest England. Bull. Geol. Soc. Am. 98,182–198.