Geomorphic Evolution of Dehra Dun, NW Himalaya: Tectonics and Climatic Coupling

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Geomorphology 266 (2016) 20–32

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Geomorphic evolution of Dehra Dun, NW Himalaya: Tectonics and climatic coupling

Swati Sinha, Rajiv Sinha ⁎

Department of Earth Sciences, Indian Institute of Technology Kanpur, Kanpur 208016, India article info abstract

Article history: The Dehra Dun is a good example of a piggyback basin formed from the growth of the Siwalik hills. Two large riv- Received 22 November 2015 ers, the Ganga and the Yamuna, and their tributaries deposit a significant part of their sediment load in the Dun Received in revised form 1 May 2016 before they enter the Gangetic plains. This work documents the geomorphic complexities and landform evolu- Accepted 2 May 2016 tion of the Dehra Dun through geomorphic mapping and chronostratigraphic investigation of the incised fan sec- Available online 6 May 2016 tions. Lesser Himalayan hills, inner and outer dissected hills, isolated hills, proximal fan, distal fan, dip slope unit, fl fi Keywords: oodplains, and terraces are the major geomorphic units identi ed in the area. Isolated hills of fan material (IHF), fi Intermontane valleys proximal fan (PF), and distal fan (DF) are identi ed as fan surfaces from north to south of the valley. The OSL Himalayan foreland based chronology of the fan sediments suggests that the IHF is the oldest fan consisting of debris flow deposits Valley fills with a maximum age of ~43 ka coinciding with the precipitation minima. The proximal fan consisting of sheet Fan deposits flow deposits represents the second phase of aggradation between 34 and 21 ka caused by shifting of deposition locus downstream triggered by high sediment supply that exceeded the transport capacity. The distal fan was formed by braided river deposits during 20–11 ka coinciding with the deglacial period. The IHF, PF and DF surfaces were abandoned by distinct incision phases during ~40–35, ~20–17, and ~11–4 ka respectively. A minor phase of terrace deposition in Dehra Dun was documented during 3–2 ka. Our results thus show that the evolutionary history of the alluvial fans in Dehra Dun was primarily controlled by climatic forcing with tectonics playing a minimum role in terms of providing accommodation space and sediment production. © 2016 Elsevier B.V. All rights reserved.

1. Introduction developed through multiple phases of aggradation and entrenchment during the late Quaternary period (Kumar et al., 2007). The develop- Tectonic and climatic processes have interacted through tightly ment of these fans is influenced by tectonic and climate factors such coupled feedbacks at variable spatiotemporal scales to shape mountain- as glacial-interglacial cycles, base-level changes, sediment supply from ous landscapes along the Himalayan belt (Seeber and Gornitz, 1983; Ori the catchment, availability of accommodation space, and degree of tec- and Friend, 1984; Lavé, and Avouac, 2000, 2001; Kirby and Whipple, tonic activity (Harvey, 1990, 2004; Silva et al., 1992; Ritter et al., 1995; 2001; Kumar et al., 2007; Suresh et al., 2007; Singh and Tandon, 2007, Harvey et al., 1999a, 1999b; Harvey and Wells, 2003; Mather and 2008). Intermontane valleys (Duns) are important elements of moun- Stokes, 2003; Mather et al., 2009; Viseras et al., 2003; Robustelli et al., tainous landscapes in the Himalaya that have developed in response 2009). Fluvial terraces also occur widely in Duns, and multiple levels to climatic fluctuations and neotectonic activities during the Quaternary of terraces have been mapped in the frontal parts of the Himalaya, period (Kimura, 1999; Singh et al., 2001; Dühnforth et al., 2007; e.g., three to five levels in the Yamuna valley (Singh et al., 2001; Dutta Goswami and Pant, 2007; Kumar et al., 2007; Suresh et al., 2007; et al., 2012) and four levels in the Ganga valley (Sinha et al., 2010). Singh and Tandon, 2008; Suresh and Kumar, 2009; Malik et al., 2010; Most rivers draining the fan surfaces are incised and therefore have de- Dutta et al., 2012). Alluvial fans, fluvial terraces and floodplains have veloped rather narrow floodplains except in the distal parts (discussed been mapped as the main landforms in the Duns (Nossin, 1971; later). In a more recent work, several piggyback basins along the Hima- Nakata, 1972; Rao, 1978; Chandel and Singh, 2000; Singh et al., 2001; layan front were examined to explore storage and release of sediments Suresh et al., 2007; Thakur et al., 2007; Singh and Tandon, 2010; Sinha from these basins and their downstream influence (Densmore et al., et al., 2010; Dutta et al., 2012). The alluvial fans in the Duns occur at 2015). The suggestion has been made that the Duns have controlled multiple topographic levels (Nakata, 1972; Suresh et al., 2002, 2007; the temporal variations in sediment flux into the Gangetic plains and Philip et al., 2009) and have been described as a piggyback system have therefore influenced the landscape development in the plains (Densmore et al., 2015). ⁎ Corresponding author. Despite several generations of geomorphic mapping and landform E-mail address: [email protected] (R. Sinha). evolution studies in the Duns in the NW Himalaya, we note significant

http://dx.doi.org/10.1016/j.geomorph.2016.05.002 0169-555X/© 2016 Elsevier B.V. All rights reserved. S. Sinha, R. Sinha / Geomorphology 266 (2016) 20–32 21 gaps in terms of terminologies, field validation, and controlling factors. hillocks, river terraces, and floodplains are the major geomorphic For example, the term ‘piedmont’ was used to describe the depositional units in this region, and the valley fills have been described as ‘Dun surfaces by Singh et al. (2001), Thakur and Pandey (2004) and Thakur gravels’ (Nossin, 1971; Nakata, 1972; Singh et al., 2001; Thakur and et al. (2007) but they do not ascribe the processes that might have Pandey, 2004; Thakur et al., 2007). The study area is confined to the shaped them. Generally, the terminology ‘piedmont’ is used for erosion- western part of the Dehra Dun between latitudes 30°20′ to 30°35′ N al and depositional landforms formed adjacent to a mountain front and longitudes 77°40′ to 78°05′ E bounded by the Tons and Yamuna riv- (Leece, 1990; Goudie, 2004). Chandel and Singh (2000) described the ers to the east and west, respectively (Fig. 1). Several ephemeral streams post-Siwalik deposits in Dehra Dun as ‘doon piedmont’ but without namely, the Suarna, Sitla Rao, Koti, Mauti, Chor Khala, and Nimi, are much process interpretation of the associated geomorphic elements. Fur- flowing in this area. These streams and several other smaller rivulets ther, a suggestion was made that the geomorphic evolution of the Dun flow SW and join the axial Asan river that in turn flows NW and involved a number of depositional and erosional phases superimposed meets the Yamuna River. The Siwalik hills to the north of the valley on each other and regulated by climatic changes and tectonics (Nakata, are highly dissected and disjointed and are separated by topographical 1972), but very limited information is available in terms of field evidence breaks that are marked by sharp turns of the streams flowing through and chronological data. Age data on different geomorphic surfaces in the the area (Thakur et al., 2007). The southern part of the valley is the Duns has been limited, and therefore a sound chronostratigraphic frame- dip-controlled slope of Siwalik Ranges and mainly composed of upper work of fans is lacking. In summary, the understanding of the evolution of Siwalik conglomerates and the boulders and gravels of quartzite and landforms and their spatial relationships is fragmentary. sandstone derived from the middle Siwalik hills (Thakur et al., 2007). We present here a detailed geomorphology of the Dehra Dun based on Table 1 shows the various geomorphic elements in the Dehra Dun mapping from high-resolution satellite images and digital elevation area proposed by various workers. Nossin (1971) mapped several large- models (DEM) coupled with field validation. Stratigraphic framework is sized fans descending from the foothills of the Mussoorie ranges. Nakata based on several lithologs from incised fan sections and a sound chrono- (1972, 1975) identified four fill surfaces and interpreted them to have re- logical data that provides an insight to the depositional history of the Dun. sulted from differential uplift within the valley. Chandel and Singh (2000) mapped Siwalik hills and post-Siwalik deposits as piedmonts that were 2. Study area and previous work further divided into high-level terraces, and upper and lower doon piedmonts. Alluvial fans are the major geomorphological units identified by The Dehra Dun is one of the largest intermontane valleys between Singh et al. (2001) and Thakur et al. (2007) in the Dehra Dun. The Main the Mussoorie Ranges of the Lesser Himalaya to the north and the Boundary thrust is the major structure that separates the Siwaliks and Siwalik Ranges to the south (Fig. 1). The Siwalik Ranges are mainly a the Lesser Himalaya. Other major structural features documented in this group of rocks and categorized as lower (mudstone), middle (sand- area include the Santaurgarh thrust (Raiverman et al., 1983), the stone), and upper (conglomerate) units. The Dehra Dun is bounded by Bhauwala thrust (Singh, 1998), the Asan and Tons faults (Thakur and major faults from all sides; the Main Boundary Thrust (MBT) to its Pandey, 2004), and the Majahaun fault (Thakur et al., 2007). north, the Himalayan Frontal Thrust (HFT) to its south, the Yamuna Fan deposits in Dehra Dun mainly consist of boulders, cobbles, and Tear Fault (YTF) to the west, and the Ganga Tear Fault (GTF) to the pebbles with a sandy and silty matrix (Nossin, 1971; Thakur, 1995). Pre- east, making it a structurally isolated block (Fig. 1). The Ganga and Ya- vious workers (Singh et al., 2001; Thakur et al., 2007) have suggested muna rivers break through the Siwalik Ranges along the transverse four major depositional units in the valley based on OSL ages. Unit A is faults, the GTF, and the YTF, respectively (Karunakaran and Rao, 1979; the oldest with ages varying from 40.3 ± 3.9 to 33.6 ± 4.7 ka and mainly Raiverman et al., 1994). The NW-SE trending intermontane valley is consisting of poorly to fairly consolidated gravels with interlayered ~80 km long and ~25 km wide. It is an asymmetrical valley with a gen- mudstone and sand beds (Thakur et al., 2007). Unit B of unconsolidated tler (0–5°) SW slope and a steeper (0–10°) NE slope. Alluvial fans, massive gravels overlies unconformably on Unit A and yields maximum

Fig. 1. (A) Location map of the Dehra Dun. (B) Study area including the western part of the Dehra Dun valley bounded by the Tons River to the east and the Yamuna River to the west. Red coloured arrows indicate flow directions of the rivers. The Main Boundary thrust (MBT) and the Santaurgarh thrust (ST) are shown with thick black lines as their activity has been observed in the field while Bhauwala thrust (BT), Majahaun fault (MJF), Asan, and Tons fault are shown with a dotted black line, as their activity has not been observed in the field. 22 S. Sinha, R. Sinha / Geomorphology 266 (2016) 20–32

Table 1 images were provided by NRSC (National Remote Sensing Center, Schemes of geomorphic mapping by different authors in the Dehra Dun area. India) and the rest of the data were downloaded from the USGS web Authors Major geomorphic units site (http://earthexplorer.usgs.gov). The LANDSAT (resolution 30 m) and SPOT (resolution 5 m) images were fused together in order to en- Nossin (1971) 1) Doon fans: high level—upper and lower group; 2) Doon fans: principal level—dissected and undissected surface; hance the spectral and spatial properties. Major geomorphic features 3) Doon fans: lower realm of coalescence; 4) Doon fans: were mapped using the LISS 3 image, fusion image of LANDSAT and terraces below principal level SPOT, topographic maps of 1:50,000 scale and the ASTER DEM. The Nakata 1) Upper Dun surface, 2) Middle Dun surface, 3) Lower Dun ArcGIS 10 software was used for mapping and geomorphic analysis. (1972, 1975) surface and higher river terraces, and 4) Middle river terraces Chandel and Singh 1) Lesser Himalaya (~2500 m), 2) dissected Siwalik hills The ERDAS imagine 8.5 software was used for image processing and (2000) (~900 m), 3) pedimented Siwaliks with thick gravel cover River Tools v. 2.4, and ‘System for Automated Geo Scientiﬁc Analyses’ (~750–450 m), 4) isolated N\\S trending Siwalik hills (SAGA) 1.2 were used for drawing the proﬁles across the valley to dis- (~850 m), 4) piedmont (upper and lower piedmonts) tinguish the geomorphic units on the basis of their relief. Fig. 2 shows the geomorphic map of the study area along with major geological structures. Table 2 provides a summary of different geomorphic units and minimum OSL ages of 29.4 ± 1.7 ka (Singh et al., 2001) and 20.5 ± mapped in this work along with corresponding terminologies used by 1.8 ka (Thakur et al., 2007), respectively. Unit C has been dated between previous workers. ~22.8 and ~10.7 ka and is mainly composed of angular to subangular Six lithologs were prepared from the exposed sections across differ- pebbles and gravels. Unit D has been dated as ~30 ka and covers the ent fan surfaces to describe the stratigraphic framework of fans. To es- northern slopes of the outer Siwalik range at the southern part of the tablish the chronology of the fan units, 11 samples were collected valley. The total thickness of fan deposits in the Dun using tube-well from the fan surfaces and terraces in the study area. Samples were col- depths, is estimated to be ~100 m in the proximal part and N300 m in lected in iron pipes and immediately sealed to prevent the exposure of the distal part (Thakur et al., 2007). The gravel units in fan deposits light. These samples were processed using optically stimulated lumines- are massive to thickly bedded, and poorly consolidated to unconsolidat- cence (OSL) dating technique at Wadia Institute of Himalayan Geology, ed. Clasts are randomly oriented and angular to subrounded (Singh India. The available OSL dates from previous literature (Singh et al., et al., 2001; Thakur et al., 2007). 2001; Thakur et al., 2007; Dutta et al., 2012) were also used, although limited information was available for these samples. Table 3 presents 3. Data and methods the OSL data generated in this work.

Geomorphic mapping of the Dehra Dun valley was carried out using 4. Major geomorphic units and fan stratigraphy satellite images, elevation data, and topographic maps. Satellite data consisted of IRS P6 LISS 3, LANDSAT image, Shuttle Radar Topography 4.1. Lesser Himalaya hills (LHH) Model (SRTM) version 4 (spatial resolution 90 m), and Advanced Spaceborne Thermal Emission and Reﬂection Radiometer (ASTER) v. 1 Lesser Himalaya hills are situated at the hanging wall of the Main (spatial resolution 30 m). The IRS P6 LISS 3 (spatial resolution 23.5 m) Boundary thrust to the north (Fig. 3A). This unit is mainly composed

Fig. 2. Geomorphic map of the Dehra Dun. The geological structures marked by the previous workers are represented as F1 and F2. F1 represents MBT—Main Boundary thrust and ST—Santaurgarh thrust which are confirmed through field evidence. F2 represents MJF—Majahaun fault, BT—Bhauwala thrust, AF—Asan fault, TF—Tons fault; their presence is not supported by any field evidence. Different geomorphic units and features are described as CB—Channel with bars, DN—Drainage network, NTH—Northern thrust sheet, DH—Dissected hills, IHF—Isolated hills of fan material, IHS—Isolated hills of Siwalik rocks, PF—Proximal fan, DF—Distal fan, UGF—Undifferentiated gravel-fill, FP—Floodplain, DS—Dip slope, T—Fill terraces. S. Sinha, R. Sinha / Geomorphology 266 (2016) 20–32 23

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

Unit Description Comparison with earlier work

Northern thrust sheet Area north of MBT with high drainage density; high relief pattern on DEM, elevation varying from 3000 to 500 m, mainly Mapped as Lesser (NTH) composed of Lesser Himalayan rocks; NDVI values are nearly ‘0’ that indicates barren land. Himalayaa,b Dissected hills (DH) Highly dissected Siwalik rocks, mainly Middle and upper Siwalik rocks (Sandstone, conglomerate); sparse vegetation at inner Same as suggested by dissected hills and thick vegetation cover at outer dissected hills; elevation varies from 1200 to 1000 m, NDVI values (−0.04 previous workers to 0.60. Isolated hills of Siwaliks Low–elevation, rounded residual hills of Upper Siwaliks; elevation ranges from 880 m to 620 m; positive NDVI values (0.040 Pedimented Siwaliks (IHS) to 0.60); distributary streams, thick vegetation cover. (hills)a,b Isolated hills of fan (IHF) Low–elevation, rounded residual hills of fan material mainly composed of massive boulders & gravels; elevation varies from Pediments GD-Ia 840 to 620 m; positive NDVI values (0.040 to 0.60); parallel drainages Piedmont Unit Ab Proximal Fan (PF) Composed dominantly of gravels and pebble with some boulders; flat surface with less vegetation; altitude varies from 840 Pediments GD-Iaa to 500 m,; NDVI values (−0.04 to +0.04) 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); Pediments GD-III, IVa slope 0–2°, parallel drainages, dissected surface, Dun gravels, Piedmont Unit Cb Undifferentiated gravel Undifferentiated gravels, identified as fill surface; elevation ranges from 560 to 460 m; few drainages, less vegetation; NDVI – values (−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 values Floodplainb negative to zero Inactive flood plain Abandoned floodplain of Asan river, cultivated areas; elevation varies from 460 to 440 m; thick soil cover; NDVI values nearly Floodplainb zero. Dip slope Dip slope gravels mainly derived from Mohand anticline; elevation varies from 680 to 500 m; slope 5° to 10°; NDVI values Piedmont Unit Da highly positive (0.40–0.60); thickly vegetated, nearly parallel drainages Terraces Fluvial terraces composed of gravels, pebbles, sand and mud; Flat surfaces, younger sediments; elevation varies from 720 to – 480 m; NDVI values are nearly zero or slightly positive

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

of Lesser Himalayan rocks, which include argillites, phyllites, quartzite, subunit is mainly composed of sandstones of middle Siwaliks and con- carbonate, and slates (Hagen, 1969; Stocklin, 1980; Valdiya, 1980). glomerate deposits of upper Siwaliks. This unit is mainly exposed at This area can easily be demarcated on the satellite images because of the eastern and western margins of the study area (Fig. 2). The con- its rugged and high relief pattern on the DEM. Slope ranges from 10° glomerate deposits are mainly made up of subrounded gravels of to 50° and altitude varies from 500 to 3000 m in this unit. The presence quartzite and mudstone derived from the Lesser Himalayan region. of active lineaments makes this area prone to landslides and debris falls The IHS unit is at a higher altitude in comparison to the IHF and (Fig. 3B) and sediments are transported to the Dun through dendritic to shows a radial drainage pattern. subdendritic drainage network. The IHF subunit is mainly composed of massive boulders, gravels, and pebbles of quartzite, sandstone, and limestone with a sandy matrix. fl 4.2. Dissected hills (DH) In general, the IHF sediments represent debris ow deposits with inter- mittent sheet flows as suggested by the dominance of disorganized The dissected hills unit is composed of middle and upper Siwalik gravels and thin layers of matrix-supported organized gravels. Isolated fi rocks that are dominantly sandstone and conglomerate, respectively. hills of fan (IHF) unit was identi ed as GD-I depositional unit by Singh ‘ ’ This unit has been mapped as ‘inner’ and ‘outer’ dissected hills based et al. (2001) and depositional unit A by Thakur et al. (2007) on its position in the valley, i.e., north and south of the Dun, respectively. (Table 2). Three lithologs, IHF1, IHF2, and IHF3, were prepared from The ‘inner dissected hills’ unit is situated on the footwall of the MBT and the exposed sections in this geomorphic unit (Fig. 4A). The IHF1 section – on the hanging wall of the Santaurgarh thrust (Fig. 3A). The elevation of is only 3 m thick with ~15 20 m of overburden. The matrix-supported ‘inner dissected hills’ ranges from 1000 to 1200 m and slope varies from organized (imbricated) gravels (80 cm thick) are present at the base 10° to 30°. The activity of the Santaurgarh thrust is manifested as deeply of the section, and the sandy matrix within the basal gravel layer yielded incised valley in the inner dissected hills and presence of large boulders an OSL age of 41.3 ± 1.2 ka. A 20-cm-thick layer of disorganized gravel upstream of the Koti (Fig. 3C), a very small stream draining the dun sur- overlies the base layer that is in turn, overlain by an ~1-m-thick layer of fi face. The ‘outer dissected hills’ constitute the SW flank of the Mohand matrix-supported organized gravels and nally by a 1-m-thick surface anticline. The elevation of the outer dissected hills ranges from 550 to soil (Fig. 5b). The IHF2 section at Dharer nala, a small tributary of the 800 m and slope varies from 5° to 30°. This unit is highly dissected by Tons River, is ~20 m thick and starts with a 50-cm-thick layer of orga- fl several small streams like the Suarna, Koti, Mauti, Tons, and many nized gravels at base suggesting channelized ow. This layer is overlain minor ephemeral streams that flow parallel to each other. by 2.5 m of a massive sand layer that yielded an OSL 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 disorganized 4.3. Isolated hills (IH) gravels with some boulders (maximum size ~50 cm). The upper parts of the IHF2 and IHF3 sections have been interpreted as proximal fan de- Isolated hills are low-elevation, rounded residual hills adjacent to posits (discussed next). the Himalayan front (inner Siwalik hills, Fig. 3A). The elevation ranges from 620 to 880 m and the slope varies from 5° to 20°. On satellite images, this unit can be identified as disjointed hills separated from each 4.4. Proximal fan (PF) other and at higher elevations compared to the alluvial fans. This unit is covered with thick vegetation (reserved forests) and yields highly The proximal fan unit occupies the area between the ‘isolated hills’ positive NDVI values (0.40–0.60; Table 2). Based on their composition, unit close to the mountain front (Fig. 3D) but extends farther down- this unit has been further divided into two subunits, namely isolated stream of the Santaurgarh thrust, ~5–10 km. The preserved fan surface hills of fan material (IHF) and isolated hills of Siwaliks (IHS). The IHS has a radius of 4–5kmandsurfaceareaof~54km2. The elevation of this 24 S. Sinha, R. Sinha / Geomorphology 266 (2016) 20–32

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

Litho-stratigraphic Sample no./Log Depth (m)a U Th K (%) Moist. cont (%) Equivalent dose (De) Gy Dose rate Age unit (ppm) (ppm) (Gy/ka) (ka)

Terrace 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 Sitla Rao Terrace 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 Chor Khala 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 ± 0.9 Distal fan FS2.2 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 (DF1) Distal 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 fan Distal fan SU-1 0.9 NA NA NA 12.6 51.25 ± 3.98 2.57 ± 0.18 19.9 ± 2.1 Proximal fan FS3.1 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 (IHF3) Proximal fan FS1.1 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 (PF1) Proximal fan DHR-2 5 NA NA NA 7.6 111.23 ± 9.84 3.69 ± 0.27 30.1 ± 3.5 (IHF2) 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 ± 1.2 Isolated hills DHR-1 9 NA NA NA 4.2 163.73 ± 23.27 3.8 ± 0.28 43.1 ± 6.8 (IHF2)

a Sample depth from surface in m. unit ranges from 500 to 840 m, and slope varies from 2° to 5° southward across the Santaurgarh fault. The exposed thickness of this unit is (Table 1). The NDVI values range between −0.04 and +0.04, suggest- ~50 m. The proximal fan unit is exposed continuously for ~1.5 km up- ing comparatively less vegetation cover with flat cultivated areas. stream of the Santaurgarh thrust along several major drainages, includ- The proximal fan unit was identified as depositional unit GD–II ing the Koti and the Suarna rivers. At the footwall of the Santaurgarh (Singh et al., 2001) and unit ‘B’ by Thakur et al. (2007) (Table 2). This thrust, this unit appears to overlie the IHF unit (Fig. 3D), and the fan unit lies unconformably over the steeply-dipping to overturned middle head has been entrenched by later fluvial processes. The proximal fan Siwalik sandstones (Thakur et al., 2007) at the hanging wall of the sediments have clasts ranging from pebbles to gravels and rarely Santaurgarh thrust but shows no evidence of offset or deformation boulders. Clasts are relatively poorly to fairly sorted but fining upward

Fig. 3. Field photographs of different geomorphic units. (A) Overview of geomorphic units from north to south. Hanging wall of the MBT is identiﬁed as ‘northern thrust sheet’.Dissected hills unit covers the area of the footwall of the MBT, and isolated hills unit are mapped as detached rounded hills south of the dissected hills. (B) ‘Northern thrust sheet’ mapped in the hanging wall of the MBT is mainly composed of the Lesser Himalayan rocks, which include argillites, phyllites, quartzite, carbonate, and slates. Slope failure is common in this unit and is the most dominant mechanism for sediment transport. (C) Field photos of ‘dissected hills’ mainly consist of middle Siwalik rocks. Presence of big boulders at the Koti River outlet in the dissected hills unit indicates tectonic activity of the Santaurgarh thrust. (D) Spatial distribution of geomorphic units of IHF and proximal fan. Deposits of the IHF unit lie unconformably on the middle Siwalik rocks at the hanging wall of the Santaurgarh thrust, and both units are unconformably overlain by the proximal fan unit. S. Sinha, R. Sinha / Geomorphology 266 (2016) 20–32 25

(normal grading), suggesting these to be sheet flow deposits. These sed- fan deposits are also exposed in the upper part of the PF2 section (0– iments were deposited in paleovalleys that were incised by at least 8 m) consisting of thick alternating layers of clast- and matrix- 70–75 m into the underlying deposits of the IHF. The axes and orienta- supported gravels. An intervening sand layer at ~3 m depth in PF2 section of these paleo–valleys are slightly offset from those of the modern tion was dated to be ~9.4 ka by Singh et al. (2001). drainage system, which means that their width cannot be observed di- Three additional OSL dates were obtained for distal fan deposits rectly (Densmore et al., 2015). The drainages flowing over the fan sur- (Table 3). The sample SU-1 came from an exposed section on the east- face are parallel to subparallel. Large boulders were observed in this ern flank of the Suarna River at a depth of 0.9 m from a location close unit toward the basin margin (Fig. 5A). to the IHF2 section (Fig. 5A) and gave an OSL date of 19.9 ± 2.1 ka Two lithologs from PF1 and PF2 sections document the stratigraphy (Table 3). The samples LIS-TOP and FS 2.1 were collected from distal of proximal fan sediments. A 6 m high PF1 section is located down- fan deposits overlying the IHF/PF deposits (Fig. 5A) and yielded OSL stream of Dharer nala, a small tributary of the Tons River. The proximal ages of 16.2 ± 1.4 and 13.8 ± 0.9 ka, respectively. Distal fan sediments fan sediments in this section consist of an organized gravel bed (base are onlapping those of the proximal fan as observed in several lithologs not exposed) overlain by a 3-m-thick disorganized boulder–gravel (IHF3, PF1, PF2) but not all the way up to the hanging wall of the layer with randomly oriented clasts and loose matrix (Fig. 4B). A thin Santaurgarh thrust; this suggests backfilling of this unit over the older sand layer above gave an OSL age of 33.8 ± 3.2 ka (Fig. 4B). An 80- fan units. cm-thick paleosol layer separates the lower proximal fan sediments in this section from the upper organized gravel layer, which was 4.6. Dip slope (DS) interpreted to be a part of distal fan deposits (discussed later). The PF2 section is a reinterpreted section (section D3 of Singh et al., 2001). The dip slope unit has been mapped on the northern flank of the This section has a massive sand layer (1 m thick) at the base for which Mohand anticline and is mainly controlled by the dip of the Siwalik an OSL age of 21.2 ± 1.2 ka was obtained by Singh et al. (2001).A Ranges (mapped as dip-controlled slope by Thakur et al., 2007). Litho- major part of this section (10.5 m thick) consists of alternating layers logically, it is mainly composed of unsorted subrounded boulders and of clast-supported and matrix-supported gravels. A 2-m-thick paleosol pebbles, predominantly of quartzite and sandstone derived from the above the sand layer represents a major discontinuity and marks the Siwalik Ranges of the Mohand anticline. The total surface area of this cessation of proximal fan deposition. unit is ~115 km2, mostly covered by thick vegetation cover The upper part of the IHF2 section (0–8 m) and the middle part of characterised by highly positive (0.4–0.6) NDVI values (Table 1). The re- the IHF3 section (6–10 m) have also been interpreted as proximal de- gional slope of this unit varies from 5° to 10°, and the elevation ranges posits onlapping the IHF sediments. In the IHF2 section, the proximal between 500 and 680 m. Based on TRMM data, Bookhagen and fan deposits consist of a massive sand layer overlain by a 3-m-thick Burbank (2006) interpreted very high annual rainfall (~100–300 cm) layer of pebbly sand and 6-m-thick layer of matrix-supported organized along the Mohand stretch because of a significant topographic high. As gravels. The upper part of this log consists of another massive sand layer a result, hillslope failures and landslides are common in this unit (4 m thick) that was dated as 30.1 ± 3.5 ka (Table 3, Fig. 4B). The sand (Fig. 5B) and they constitute the dominant sediment transport process- layer is overlain by a 2-m-thick layer of clast-supported organized es in the area (Barnes et al., 2011). The OSL age of ~30 ka (Thakur et al., gravels, and the section is capped by a 2-m-thick surface soil. In the 2007) suggests a major unconformity between the underlying Siwalik IHF3 section, the proximal fan deposits are made up of a sand layer strata (age ~0.5 Ma; Sangode et al., 1996) and the dip slope unit. (2-m-thick) with a date of 21.2 ± 1.2 ka (Fig. 4B) and a thin layer (1 m) of the imbricated gravels, suggesting a brief period of channelized 4.7. Channel belt flow. The overlying paleosol layer (80 cm thick) marks a minor discontinuity in the sequence, and the overlying sediments were interpreted Channel belts of the major rivers such as the Koti, Mauti, Sitala Rao, to be a part of distal fan deposits (discussed next). 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 4.5. Distal fan (DF) ~20 to 25 km from their point of origin in the Lesser Himalaya to their confluence with the axial river, the Asan (Tons). The Asan (Tons) origi- The distal fan has been mapped farther south of the proximal fan. nates from southern slopes of the Mussoorie hills and drains for ~50 km The southern stretch of this unit is bounded by the Asan River. The distal before meeting the Yamuna. Channel belts of most of the rivers are quite fan unit is a near-planar surface with elevation varying from 480 to narrow (b0.5 km) in the upstream reaches, but they become wider (1 to 700 m sloping b2° southward. The observed fan radius is ~10–17 km, 1.5 km) in downstream reaches. Small streams such as the Nimi, Dharer, and the total surface area is ~131 km2. This unit consists of poorly Ghulata, Udmadi Rao, and Chor khala originate within the valley, mainly sorted, angular to subangular pebbles with rare boulders (depositional from the detached residual hills (IH unit). These are ephemeral streams unit ‘C’ of Thakur et al., 2007). Singh et al. (2001) identified the same and most of them are b15 km long. Large boulders were noted in the unit as depositional unit GD III and GD IV (Table 2) with pedofacies III. channel belt of the Kosi and Mauti rivers in their upstream reaches. In The exposed thickness of this surface varies from ~15 m in the northern the downstream reaches, gravels and pebbles constitute a major part part to ~5 m in the southern part. The NDVI values vary from −0.04 to of channel sediments. +0.40 for this unit, suggesting barren lands or settlements with some areas of thick vegetation cover. The topography of this unit is highly un- 4.8. Floodplain (FP) dulating owing to dissection by several minor streams. A ~10-m-thick exposed section (DF1) on the western bank of the The major rivers draining the study area, the Yamuna, Suarna, Sitala Sitala Rao has a 4.5-m-thick layer of disorganized gravels at the base Rao, and Asan, have prominent floodplains. The general slope of the overlain by a 1-m-thick sand layer that was dated as 14.4 ± 0.6 ka floodplains ranges from 0° to 1° and the elevation ranges from 420 to (Fig. 5B). The sand layer is overlain by a 1.5-m-thick clast-supported or- 660 m. The floodplain unit was demarcated on the satellite images on ganized gravels, and then by a 1-m-thick matrix-supported organized the basis of image characteristics such as high moisture content and gravel. In the IHF3 section, the uppermost (0–5.5 m) part of the se- NDVI values (negative to zero; see Table 1). The floodplain along the quence has been interpreted as distal fan deposits consisting of two Asan River is 0.5–2.5 km wide but is much narrower (600–900 m) layers of matrix-supported organized gravels, each ~1 m thick, separat- along the Suarna River (Fig. 5C). The floodplain unit is composed of ed by a thin sandy layer. The section is capped by sandy soil (1 m thick) sand and silt and little mud with some pedogenic modification and in- that is highly disturbed owing to the presence of thick vegetation. Distal tercalated with pebbles. At the western end of the study area, the 26 S. Sinha, R. Sinha / Geomorphology 266 (2016) 20–32 S. Sinha, R. Sinha / Geomorphology 266 (2016) 20–32 27

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 the base 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 the river 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 of the river, ~1 m below the surface, yielded an OSL age of 3.53 ± 0.21 ka.

Yamuna has a well-developed floodplain, and the width varies from streams flowing from the northern flank of the Mohand anticline that 0.5 km in the upstream to 3 km in downstream reaches. join the Asan River to the south. The formation of this asymmetric terrace along the Asan may be related to the uplift of the Mohand anticline. 4.9. Terraces (T) Another small symmetric depositional terrace surface, ~0.5 km long, ~0.2 km wide, and ~1 m high, was identified along the Chor Khala One set of paired fill terraces were mapped along the Sitala Rao and River (Fig. 5D), a small tributary of the Asan River. An OSL date of Suarna rivers. Terraces are cultivated areas with elevation ranging from 3.5 ± 0.2 ka was obtained for a sample from this terrace (Table 3). 480 to 720 m. Terraces are mainly composed of gravels and pebbles in sandy matrix. The NDVI values of these surfaces are nearly zero or slightly positive because of the presence of barren land and cultivated 4.10. Undifferentiated gravels (UGF) areas. Terraces along the Sitala Rao are ~5 km long and ~0.5–0.8 km wide. We obtained an OSL age of 0.9 ± 0.2 ka from a sample taken This unit, composed mainly of gravels, is flat and occupies a total from this terrace (Table 3). Terraces along the Suarna River are ~4 to area of ~200 km2 north of the Asan river and east of the Tons river. In 5 km long and ~0.5 km wide, located ~2 to 3 m above the river bed the surface profile, there is no major break between distal fan unit and and are flat with sparse vegetation cover (Fig. 5C). An approximately undifferentiated gravels unit. A large part of this unit is dotted with cul- ~30-km-long and 0.7 to 2.5 km wide asymmetric terrace was identified tivated land and settlements including the city of Dehradun. Although it along the southern bank of the Asan river. This terrace is bordered by has been mapped as a fairly continuous unit, it is quite scattered and dis- the dip slope (DS) unit at the south and is cut by several ephemeral jointed owing to human interventions.

Fig. 4. (A) Locations of various lithologs and OSL samples on the satellite image. (B) Lithologs and chronology of all sections exposing different fan units. IHF1, IHF2, and IHF3 logs document IHF deposits that are interpreted as debris ﬂow deposits with maximum OSL ages of ~43/41 ka. The upper parts of the IHF2 and IHF3 sections have been interpreted as proximal and distal fan deposits respectively, yielding younger ages. Lithologs PF1 and PF2 document proximal fan deposits with ages of 33–21 ka. The PF1 and PF2 sections record distal fan deposits in the upper parts. Litholog DF1 is exposed at the west bank of Sitala Rao and has 4.5-m-thick unit of disorganized gravels. Thin (~1 m) sandy layer yields OSL age of 14.4 ± 0.6 ka for distal fan deposits. 28 S. Sinha, R. Sinha / Geomorphology 266 (2016) 20–32

5. Discussion thrust. This backfilling of the proximal fan deposits could have been triggered because of tectonic influences (e.g., increase in slip rate; Bull, 5.1. Tectonogeomorphic evolution and depositional history of the Dehra 1961, 1964; DeCelles et al., 1991; Gordon and Heller, 1993; Mellere, Dun 1993; Allen and Densmore, 2000). The increase in slip rate provides accommodation space and the younger fan unit can onlap the older units The IHF unit is a steeply dipping fan surface and is mainly composed (Densmore et al., 2007). Backfilling can also be influenced by hydraulic of clast- and matrix-supported boulders to pebble conglomerate. The conditions such as less water availability or low water-sediment ratio clasts are poorly sorted and randomly oriented suggesting dominant that can lead to the onlapping of the younger unit over the older one debris flow deposits (Blair and McPherson, 1994; Singh et al., 2001; close to the mountain front (Goudie, 2004; Densmore et al., 2007; Nichols, 2009). The deposition of isolated hills of fan IHF unit occurred Hamilton et al., 2013). However, as no deformation was observed on during ~43–33 ka coinciding with the precipitation minima around the fan surfaces in the Dun and the widespread filling occurred around 40 ka (Prell and Kutzbach, 1987). We argue that the availability of sed- the LGM time, we argue that climatic factors have had a dominant role iments in the system was related to reactivation of MBT during in back filling of the proximal fan unit. 70–55 ka (Banerjee et al., 1999)and50–35 ka (Singhvi et al., 1994; The presence of boulders in the distal part of the fan unit (Fig. 4A) Singh et al., 2001) that led to the deposition of thick strata of IHF has been debated and alternative hypotheses have been proposed. (≥41 ka) in the Dun (Fig. 6). Low transport capacity caused by the pre- Kumar et al. (2007) and Giano (2011) interpreted the presence of boul- cipitation minimum during this period resulted in deposition close to ders in fan deposits toward the basin margin because of high precipita- the mountain front. tion in the catchment. On the contrary, Singh et al. (2001) suggested The proximal fan unit primarily consists of matrix- and clast- that the development of the pediments 1 and 2 (IHF in the present supported pebbles to gravels with some boulders having loose contact work) caused by the activity of the Bhauwala thrust and headward ero- with matrix that suggests mud flow or sheet flow deposits (Blair and sion of the streams. They also suggested that the pedimented Siwalik McPherson, 1994; Singh et al., 2001) with minor debris flows. The prox- (IHS in the present work) and fan area were uplifted caused by the ac- imal fan unit is the second phase (34–21 ka) of major aggradation in the tivity of the Bansiwala thrust (Asan fault according to Thakur et al., Dun that coincides with a period of high sediment production from the 2007). In the hanging wall of the Santaurgarh thrust, fan deposits Himalaya (Sharma and Owen, 1996; Taylor and Mitchell, 2000; Pant were observed lying unconformably over the steeply dipping to et al., 2006). This is also the period of high precipitation that led to overturned middle Siwalik sandstones (Thakur et al., 2007), but no ev- high intensity floods in the foreland induced by high solar insolation idence was recorded for any deformation in the fan units across the at ~31 ka (Goodbred, 2003). High sediment supply (Lecce, 1990) Santaurgarh thrust. We therefore favour the climatic interpretation and an increase in water-sediment ratio (Ballance, 1984; Wells and (high precipitation) for such deposits. Harvey, 1987; Harvey and Wells, 2003; Densmore et al., 2007; The distal fan unit consisting of poorly sorted, angular to subangular Nichols, 2009) during this period were the primary reasons for transi- pebbles have been interpreted as braided stream deposits and overbank tion from debris flow deposits (IHF unit) to sheet flow deposits (PF deposits. The distal fan unit (20–11 ka) represents the third phase of de- unit). The elevation difference of ~100 m between the IHF and PF position in the Dun. Sheet-like fan deposits indicate high precipitation units (Table 2) suggests an incision of the IHF unit prior to the deposi- in the catchment area (Densmore et al., 2007). The timing of distal fan tion of proximal fan unit. deposition coincides with the post-LGM period that corresponds to The onlapping of the proximal fan deposits over the IHF deposits high sediment supply from the hinterland because of monsoonal inten- (lithologs IHF2 and IHF3) at the hanging wall of the Santaurgarh thrust sification (Overpeck et al., 1996) and to extensive slope failures and hill suggests widespread deposition of the proximal fan unit during the slope sediment transport (Pratt et al., 2002). The lithologs PF1 and PF2 LGM and eventually backfilling toward and across the Santaurgarh clearly show that the distal fan sediments onlapped the proximal fan

Fig. 6. Thematic section across the Mohand anticline and the Lesser Himalaya. Major geomorphic units and structures in the Dun are marked on the section. Isolated hills of fan (IHF), proximal fan (PF), and distal fan (DF) are the three major fan surfaces with the ages N41, 34–21 and 20–11 ka, respectively. Dissected hills unit is mapped at the footwall of the MBT and isolated hills unit is mapped as detached rounded hills south of the dissected hills. Dip slope geomorphic unit was mapped on the NW flank of the Mohand anticline. S. Sinha, R. Sinha / Geomorphology 266 (2016) 20–32 29 deposits. This again suggests backfilling of distal fan deposits (Fig. 6)but precise timing of deposition and abandonment of this unit could not not up to the hanging wall of the Santaurgarh thrust. We argue that such be ascertained. The proximal fan formed because of continuous deposi- backfilling is related to water-sediment ratio influencing the transport tion from 34 to 21 ka and developed between isolated hills. Field evi- capacity of the river systems (Goudie, 2004; Densmore et al., 2007; dences suggest incision of isolated hills and refilling of the valley. The Hamilton et al., 2013). activity of the Santaurgarh thrust before 23 ka (Singh et al., 2001)pro- During the early to mid-Holocene period, minor terrace deposits vided high sediment availability during this period and led to continu- dating 11 ka (Singh et al., 2001) and 3 ka (present work) have been doc- ous deposition of PF until 21 ka. The distal fan (stage III) formed umented along the rivers in the Dun. This period was the beginning of owing to further southward shift of depositional locus after the aban- the incision phase in the Dun because of intense monsoonal activity donment of the proximal fan and fanhead entrenchment. Deposition (Singh et al., 2001). An incision phase started in the Duns at ~12/10 ka of the distal fan sediments (~20–11 ka) may have progressed as and has continued until present (Singh et al., 2001; Suresh and backfilling over the proximal fan deposits (34–21 ka), but no distal fan Kumar, 2009). No deposition was recorded after 3 ka except a minor re- deposits are observed over the hanging wall of the Santaurgarh thrust. cent aggradational terrace (OSL age 0.9 ka, sample ST-1) along the Sitala A minor depositional phase is also documented in the intermontane Rao (Fig. 4A). basin during 3–1 ka (stage IV) that led to thin depositional terraces along the modern rivers. 5.2. Conceptual model All four depositional units, namely isolated hills of fan, proximal fan, distal fan, and younger depositional terraces, are separated by Fig. 7 shows the sequential development of the different fan units in strong incision phases (Fig. 7). The timing and duration of the inci- the Dehra Dun. The deposition of the IHF unit (Stage I) began at ~43 ka sion phases are not precisely constrained, but these must have oc- and probably ended around ~33 ka. The reactivation of MBT during curred after the deposition of fan units, at ~33 ka for IHF, ~20 ka 75–65 and 50–35 ka (Singhvi et al., 1994) provided high sediment avail- forPF,and~10kaforDF.Earlierwork(Densmore et al., 2015)esti- ability and low precipitation minima (Prell and Kutzbach, 1987), mated that these incision episodes contributed a significant amount resulting in low transport capacity and deposition of sediments close of sediment discharge into the plains that were ca. 1–2% of the to the mountain front. The geomorphic expression of this fan is not present-day suspended load of the Ganga and Yamuna rivers that well preserved because of several cycles of incision. Therefore, the traverse the margin of the Dun.

Fig. 7. Evolutionary model of the Dehra Dun fills. (I) IHF deposits formed during 43–33 ka, and the reactivation of MBT during 75–65 and 50–35 ka provided high sediment availability and low precipitation minima, resulting in low transportation capacity that resulted in sediments deposited close to the mountain front. (II) Proximal fan formed because of continuous deposition from 34 to 21 ka and developed between isolated hills; field evidences suggest incision of IH and refilling of the valley. The activity of the Santaurgarh thrust before 23 ka (Singh et al., 2001) provided high sediment availability during this period and led to continuous deposition of PF until 21 ka. (III) Distal fan (20–11 ka) formed when the depositional locus shifted southward owing to fanhead entrenchment of the proximal fan. (IV) A dominant incision phase (b5 ka) in the Dun with deposition of minor younger terraces during 3–1 ka. (DD—Dehra Dun; PD—Pinjaur Dun, KD—Kiarda Dun, PaD—Parduni basin, CD—Chitwan Dun). 30 S. Sinha, R. Sinha / Geomorphology 266 (2016) 20–32

5.3. Comparison with other duns (intermontane valleys) and regional Alaknanda–Ganga system in the Lesser Himalaya indicate that aggrada- correlation tion and incision phases between 63 and 11 ka resulted primarily because of glaciation–deglaciation processes (Ray and Srivastava, 2010). The OSL ages suggest that the fan development in western Dehradun Mass wasting (landslides) and sheet flow processes during the transi- started at ~43 ka and ceased around 10 ka. Aggradational phase I (43– tion between glacial and nonglacial periods (Barnard et al., 2006a, 33 ka) that corresponds to formation of the IHF unit is comparable to 2006b; Dortch et al., 2009; Seong et al., 2009) also contributed to the the first aggradation phase in Yamuna valley (~37–24 ka) between sediment input in the Duns (Chandel and Singh, 2000; Singh et al., the MBT and the HFT (Dutta et al., 2012). An aggradational phase was 2001). However, it has been suggested that tectonic activities along lon- also documented during ~49–25 ka at various upstream sites in the gitudinal as well as transverse faults have continuously modified the Ganga basin (Srivastava et al., 2008; Ray and Srivastava, 2010). Fan ag- landforms in the Duns (Kimura, 1994, 1999; Chandel and Singh, 2000; gradation in the Pinjaur Dun was also documented from 72 to 24 ka Singh et al., 2001; Virdi et al., 2006; Philip and Virdi, 2007; Singh and (Kumar et al., 2007; Suresh et al., 2007) but without any incision Tandon, 2008; Philip et al., 2009). phase as recorded in Dehra Dun (Fig. 7). Phase II (~34–21 ka) of proximal fan deposition correlates with de- 6. Conclusions positional phases in the Soan Dun in NW Himalaya during ~36–29 ka (Suresh and Kumar 2009) and falls within the fan aggradation phase Geomorphic and stratigraphic investigations in the Dehra Dun in (~72–24 ka) in the Pinjaur Dun in NW Himalaya (Kumar et al., 2007; NW Himalaya have suggested that at least three phases of fan sedimen- Suresh et al., 2007). Fan aggradation phases in the Kiarda and the tation occurred during the late Quaternary period (I) 43–33 ka, (II) 34– Parduni basins in NW Himalaya –recorded from ~34 to 19 ka and 21 ka, and (III) 20–11 ka followed by minor terrace formation during 3– from ~27 to 20 ka, respectively (Philip et al., 2009) – are also correlat- 1 ka. Each depositional phase was followed by fanhead incision, aban- able to this phase (Fig. 7). In Chitwan Dun in the Narayani basin, south- donment of the active depositional locus and shifting of the depocenter ern Nepal, no absolute ages of the fill deposits are available, but Kimura toward the basin margin. Climate change (high precipitation) was the (1995) suggested ages of the depositional units as ~26–16 and b10 ka primary reason for the transition from debris flow deposits to sheet separated by an incision event. flow and stream flow deposits in the Dun. While the evolutionary Aggradational phase III (20–11 ka) represented by distal fan deposi- history of the alluvial fans in the Dun was influenced by tectonic tion in Dehra Dun, correlates with the second aggradation phase activities, particularly in terms of providing accommodation space, the (15–12 ka) in the Yamuna valley between the MBT and the HFT aggradation and entrenchment of fans were controlled by climatic (Dutta et al., 2012). Aggradation in the Ganga valley between MBT perturbations. and HFT was also documented at ~11 ka followed by incision during – 11 9.7 ka (Sinha et al., 2010). A phase of aggradation was documented Acknowledgements at several upstream sites in the Ganga basin during 18–11 ka (Srivastava et al., 2008; Ray and Srivastava, 2010). A minor depositional phase was This work was supported by a research grant from the Department also observed in the Pinjaur Dun during 16 ka (Suresh et al., 2007). of Science and Technology, Government of India (SR/S4/ES-415/2009) – Minor terrace deposits during 3 1 ka in the Dehra Dun (aggradational including the studentship to SS and we thankfully acknowledge this phase IV) correlates with a brief depositional event in the Yamuna val- support. A part of the field investigations and OSL dating were support- – ley during (~5 1 ka) (Dutta et al., 2012) and the Pinjaur Dun during ed from a grant from UKIERI (IND/CONT/07-08/199), British Council 4.5 ka (Fig. 7)(Suresh et al., 2007). Fan aggradation ceased ~11 ka ago that was crucial for this research. Discussions with Alexander in the Dehra Dun (Singh et al., 2001) and so did the second phase of Densmore, Jason Barnes, Sampat Tandon, Malay Mukul, Vikrant Jain, fan deposition in the Kangra region around 10 ka (Suresh and Kumar, and Vimal Singh during field work and project review meetings were 2009). most useful to develop the ideas presented in this paper. We thank all It has been observed that major fan aggradational and incision the reviewers for their detailed comments that helped to improve the phases in the Duns along the Himalayan arc (Kumar et al., 2007; manuscript significantly. 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