Geomorphology 266 (2016) 20–32 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph 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 sur- faces 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 evolu- tionary 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 pied- monts. 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.