Late Pleistocene-Holocene morphosedimentary ... - Springer Link

Int J Earth Sci (Geol Rundsch) (2013) 102:1967–1984 DOI 10.1007/s00531-013-0871-y

ORIGINAL PAPER

Late Pleistocene-Holocene morphosedimentary architecture, Spiti River, arid higher Himalaya Pradeep Srivastava • Yogesh Ray • Binita Phartiyal Anupam Sharma

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Received: 16 May 2012 / Accepted: 6 February 2013 / Published online: 1 March 2013 Ó Springer-Verlag Berlin Heidelberg 2013

Abstract The Spiti River drains the rain shadow zone of western Himalaya. In the present study, the fluvial sedimentary record of Spiti valley was studied to understand its responses to tectonics and climate. Geomorphic changes along the river enable to divide the river into two segments: (i) upper valley with a broad, braided channel where relict sedimentary sequences rise 15–50 m high from the riverbed and (ii) lower valley with a narrow, meandering channel that incises into bedrock, and here, the fluviolacustrine sediments reside on a bedrock bench located above the riverbed. The transition between these geomorphic segments lies along the river between Seko-Nasung and Lingti villages (within Tethyan Himalaya). Lithofacies analyses of the sedimentary sequences show six different lithofacies. These can be grouped into three facies associations, viz. (A) a glacial outwash; (B) sedimentation in a channel and in an accreting bar under braided conditions; and (C) formation of lake due to channel blockage by landslide activities. Seventeen optically stimulated luminescence ages derived from ten sections bracketed the phases of river valley aggradation between 14–8 and 50–30 ka. These aggradation phases witnessed mass wasting, channel damming and lake formation events. Our record, when compared with SW monsoon archives, suggests that the aggradation occurred during intensified monsoon phase of MIS 3/4 and that proceeded the Last

P. Srivastava (&) Y. Ray Wadia Institute of Himalayan Geology, 33 GMS Road, Dehradun 248001, India e-mail: [email protected] B. Phartiyal A. Sharma Birbal Sahni Institute of Palaeobotany, 53-University Road, Lucknow 226007, India

Glacial Maxima. Thus, the study reports monsoon modulated valley aggradation in the NW arid Himalaya. Keywords Arid NW Himalaya Spiti River Fluvial archive OSL dating Paleoclimate

Introduction Erosion and sedimentation processes of rivers sculpt landscapes and form terraces and alluvial fans that together indicate the climate and tectonic activities in mountains and its foreland. Interplay of tectonics and climate defines the time scales and locations (hotspots) of sediment generation and deposition. Spatially heterogeneous distribution of rainfall, erosion and mass removal potentially induces non-uniformity in geomorphic development. During the evolution of the southern mountain front, sedimentation and erosional processes in the Himalayan region were dominated by the tectonics arising from south verging regional scale thrusts like the Main Central Thrust (MCT), Main Boundary Thrust (MBT) and Himalayan Frontal Thrust (HFT) and by climatic conditions. The region between the hanging wall of the MCT and the southern region of Indus suture zone is located in the lee side of High Himalaya and therefore is rather characterized by dry climate. This region is also deformed by several normal faults. The Indus suture zone itself is arid. This heterogeneity potentially induces variability in landscape evolution across Himalaya. Rivers erode and carry sediment from the High Himalaya to the foreland and finally to the oceans and carrying signals of climate–tectonic impacts on landscape development. Therefore, understanding of fluvial systems in different climate and tectonic regime is cardinal to the understanding of geomorphic evolution of Himalaya vis-a`-vis climate and tectonic changes.

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Several studies on the landscape development, erosion and depositional processes, their time scales and forcing factors such as tectonics and/or climate have been reported (Thiede et al. 2004; Wobus et al. 2005; Srivastava et al. 2008; Ray and Srivastava 2010). However, Himalayan region beyond the spatial limits of monsoon is not well studied. Drier Himalaya has little vegetation and receives average annual rainfall of 50 mm. During abnormal monsoon years, this rainfall episodically is up to 15 mm/day and leads to heavy hill slope erosion, landslides and damming of river channels (Korup et al. 2006). In Sutlej and Spiti River valleys (western Himalaya), evidence of land sliding and subsequent channel damming during Late Pleistocene-Holocene intensified monsoon phase has been reported (Bookhagen et al. 2005a; Phartiyal et al. 2009a, 2009b). Sutlej valley recorded a fivefold increase in sediment generation during phases of intensified monsoons (Bookhagen et al. 2005a, b). These observations suggest that arid and semi-arid regions of the Himalaya with sparse vegetation, steep hill slopes and channel networks are sensitive to climatic variability (Montgomery and Dietrich 1992; Tucker and Slingerland 1997). Spiti River valley in the western Himalayan zone is traversed by several active normal faults (e.g. Kaurick Chango Fault; Bhargav et al. 1978; Bhargava and Bassi 1998). Therefore, climate is not the only factor that affects the sedimentary deposit and landscape architecture of this valley. The present study reconstructed the geomorphic and sedimentological evolution of the Spiti River valley via the morphosedimentary record coupled with optically stimulated luminescence (OSL) dating. Sedimentological analysis of provided lithofacies associations suggestive of mass wasting, channel damming and formation of lakes helped understand the phases of higher erosion and sediment transport, and the role of neotectonic activities, in the evolution fluvial landscape.

General geology The Spiti valley is an area where Tethyan sediments from Neoproterozoic to Cretaceous are exposed (Bhargava and Bassi 1998; Sinha 1989 and references therein). The Palaeozoic rocks are mostly splintery shale, sandstone, limestone and metasediments (e.g. quartzites, marble, slate). The Mesozoic rocks are dominantly limestone (Kioto limestone-Jurassic), shale (Spiti shale-Triassic?) and sandstone (Giumal sandstone-Cretaceous). At the northern end of the Spiti valley, the Tso Morari Crystallines of age *400 Ma comprises granitic gneiss, gneiss and schist occur (Steck et al. 1998; Jain et al. 2003; Leech et al. 2005).

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Int J Earth Sci (Geol Rundsch) (2013) 102:1967–1984

Structurally, Spiti River valley is located SW of the Karakoram Fault System (KFS), in a pull-apart basin lying between NW–SE trending, right lateral, strike-slip Karakoram Fault System and high-angle faults near the southern boundary of Tethyan Himalaya (Ni and Barazangi 1985; Bhargava 1990; Mazari and Bagati 1991). The Tethyan Himalaya falls between the MCT (in the south) and the Zanskar thrust (Great Counter Thrust) in the north. The NNE–SSW trending Kaurik-Chango fault and faults associated with the Leo-Pargil Horst control active tectonics of the region (Thiede et al. 2006; Hintersberger et al. 2010). The Kaurik-Chango normal fault zone (KCnf) strikes north-northeast, dips up to 808 west and comprises a cataclastic zone along the western flank of the Leo Pargil Dome (Bhargav et al. 1978). The faults cut the hanging and footwall rocks of the Leo Pargil Dome and offset river terraces of quaternary age (Thiede et al. 2006). Kinematic data from this fault zone indicate dip-slip normal faulting and an E-W extension. This was also supported by focal mechanism data for Kinnaur earthquake of 1975 that occurred along this fault indicated dominant normal dipslip displacement (Molnar and Chen 1983). Seismic activity along this structure is visible in faulted lacustrine and fluvial units dated to between 26 and 90 ka (Singh et al. 1975; Mohindra and Bagati 1996; Baneerjee et al. 1997). Morphotectonic parameters such as basin asymmetry and drainage anomalies also indicated that the KCnf has been neotectonically active (Joshi et al. 2010).

Climate and sediment generation The Spiti valley is located above the tree line (3,000 m amsl), between 31 and 33°E and 77–79°S (Fig. 1), with a few shrubs on the valley floor. This region is in the monsoon rain shadow, behind the High Himalaya (Fig. 2). The area receives average of 50 mm of annual rainfall (excluding snow melt component) and \200 cm of snowfall per annum. The extreme temperatures are -25 °C (winters) and 15 °C (summers). Two precipitation regimes operate, viz. the southwest (June to September) and the winter monsoon from western disturbances (November to February). During the past, flash floods in the Sutlej River are reported, with instances of water level rising to 12 m above the normal monsoon flow. During such abnormal monsoon years, moist air bypassed orographic barriers and reached arid regime (Bookhagen et al. 2005b). Such events erode the slopes and fill the river valleys with sediments as has been observed in other orogens (Trauth et al. 2003; Bookhagen et al. 2005b). Spiti River derives sediments from relict glacial moraines and hill slope weathering via freezing and thawing processes (Keiffer and Steinbauer 2012) that produce a


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Fig. 1 a The study area is marked by blue rectangle, black rectangle marks the region used to prepare the swath profile as shown in Fig. 2. b Lithotectonic division of Himalaya, ITSZ, Indus Tsangpo Suture Zone; STD, South Tibetan Detachment; MCT, Main Central Thrust; MBT, Main Boundary Thrust; and HFT, Himalayan Frontal Thrust, red rectangle marks the study area. c Spiti River (in blue) valley showing the distribution of relict fluvial deposits in lower and upper Spiti River valley. d Braided pattern of the river. Locations shown in pink are the sites where sedimentary sections are studied and dating samples are collected and those with red circle are the sites where palaeolandslides are located

huge amount of loose debris to form debris cones that supply sediment to the main channel during anomalous rainfalls (Fig. 3a). During intense snow and its subsequent melting, pore pressure increase in the debris leads to debris flow that form steep sedimentary cones (Fig. 3b). This is an important source of sediment supply to the main channel. Further only occasional debris flow events are not always capable to dam the channel of the size like the Spiti River.

Geomorphology of the Spiti River Spiti River is about 150 km long and originates from Nogpo-Topko glacier, near Kunzum La (4,551 m) as the Taktsi stream. This joins Pagnu and Kibji rivers, and the system thereafter is called Spiti River. In its entire course, the river descends up to *1,800 m, that is, an average slope of 17 m/km. Longitudinal profile, slope gradient (Hack profile) and steepness indices of the river show sharp knick points when the channel crosses active normal faults, for example, at Mane and Kaurik-Chango (Phartiyal and

Kothari 2011; Anoop et al. 2012). Initially, from Losar to Mane, the river flows E-W, as a braided stream, in a U-shape valley, takes a gentle right angle turn and thereafter flows linearly in a SE direction in the axial plain of the Spiti anticline and then joins the Sutlej river at Khab. The Spiti River has a catchment of *6,300 km2, with Pin, Lingti, Parachu as its major tributaries. Based on the channel characteristics and disposition of the relict deposits, the whole valley was divided into the upper and lower Spiti valley (Fig. 1b). In the upper valley, between Losar and Lingti, the river is braided and the valley walls are abutted by relict fluvial and lake deposits, active fans, debris cones and landslide sediments. The lower valley, between Lingti and Khab, is characterized by a meandering and incised channel (Fig. 4). This segment has incised gorges with relict lacustrine and fluvial deposits residing on bedrock which is 10–130 m above the river level. At places, the river has incised into the bedrock with a sinuous course (Fig. 4). The tributaries and other first- and second-order streams join the main river at right angles suggestive of tectonic/ structural control. The general NW–SE course suddenly

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Fig. 2 a Map showing the change in elevation from the frontal Himalaya to the Spiti valley, black rectangle marks the area used to prepare the Swath profile. b Swath profile (250 9 100 km) showing black line as mean elevation derived from Shuttle Radar Topographic Mission DEM (SRTM 2000), and blue line marks mean precipitation derived from Tropical Rainfall Measurement Mission (TRMM)


rainfall data (Bookhagen, in review), shading denotes ±2r values showing the maximum and least value along the transect. The geomorphic and rainfall profile suggests that the influence of the SW monsoon largely remains south of the Southern Himalayan Front, keeping the Spiti River valley arid

Fig. 3 a Steep sedimentary cones bordering the river channel. b The fans often collapse and slide down as debris flows due to high intensity rains, sometimes blocking the river

changes to almost N–S direction along the KCnf all the way to junction with the Sutlej River at Khab. Apart from this features like unpaired and tilted terraces, deep gorges and knick points, basin asymmetry is also suggested as the manifestation of neotectonic movement along the KCnf in the area (Joshi et al. 2010; Phartiyal and Kothari 2011).

Methodology The geomorphic study used Survey of India (SOI) topographic maps and field surveys. SOI Toposheets, altimeter

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and hand-held global positioning systems were used to determine the heights of terrace surfaces. Thickness of the alluvial fill was measured using tapes and rods. Lithofacies of fluvial fills, fans and palaeolake sequences were documented by a careful observation of grain size, colour, degree of bioturbation, matrix percentage, physical structures, lateral geometry and bounding contacts of the individual units. Gravel diameter and matrix percentage estimation used field observation using 1-m2 grids, and several outcrops were examined. The discussion here follows a type section. The morphological details were after careful examination of SOI topographic maps, images from


Fig. 4 Meandering reach of the Spiti River from downstream Lingti in lower Spiti valley. Note that the meandering channel has incised a narrow gorge, with entrenched meanders. The river incises its meanders thereby increasing sinuosity in response to episodic uplifts

Google Earth and field observations. Sample collection for dating was done in steel pipes, and generally sandy alluvial units were sampled. Optical dating was the preferred technique for the dating of the various geomorphic units. The technique relies upon the fact that process of erosion and transport, daylight exposures of minerals constituting the sediments, resets their latent luminescence to a near zero residual value. On burial day, light exposure ceased and re-accumulation of luminescence due to irradiation from the natural radiation field (arising from the decay of natural radioactivity) occurs. This continues unabated till excavation, and the stored luminescence is proportional to the radiation exposure and hence the burial age (Aitken 1998). There is encouraging evidence of zeroing during fluvial transport of quartz from the foothills of the NE Himalaya (Mukul et al. 2007; Srivastava and Misra 2008) which is facilitated by dry conditions, lack of vegetation cover and enhanced UV flux at higher altitudes. A total of 21 samples were collected from sections along the river (Fig. 1). Samples SP-2 to SP-14 were from the upper valley and SP-14-21 were from the lower valley. The details of the sections and stratigraphic positions of the samples are given in Table 1. Quartz fraction was extracted by treating the samples sequentially with HCl, H2O2 followed by heavy liquid separation using sodium polytungstate (density = 2.58 g/ cm3). These grains were then sieved to extract the 90–150 lm fractions and etched using 40 % HF for 80 min followed by 12 N HCl treatment for 40 min, the alpha irradiated skin, residual feldspars and insoluble fluorides. Purity of quartz vis-a`-vis feldspar contamination was tested

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using infrared stimulated luminescence. Quartz grains were mounted on stainless-steel discs using Silko-Spray silicone oil, and 15–20 aliquots of 9 mm diameter were prepared for luminescence analysis. Low luminescence sensitivity in some samples necessitated the use of large aliquots. Luminescence measurements were made in a Riso TL/ OSL-15 system with an array of blue LEDs for stimulation. The signal was recorded through a combination of BG39 ? U-340 filter for 40 s at 125 °C. A 90Sr/90Y beta source delivering a dose rate of 6.7 Gy/min was used for irradiation. Palaeodose estimation was carried out using a 5-point single aliquot regeneration protocol of Murray and Wintle (2000). Three regeneration dose points were used to construct dose growth curves and two points to check the recuperation effect and for sensitivity corrections (recycled point), respectively. A preheat of 220 °C for 10 s for natural and regeneration doses was used. The dose growth curves were selected having \10 % variation in recycling ratio. The quartz examined shows a typical shine down curve with a linear growth curve (Aitken 1998). The initial part (2 s of a 100 s exposure) of a typical shine down curve of quartz was used for analysis. The uranium (238U), thorium (232Th) and potassium (K) concentrations were measured by X-ray fluorescence. The cosmic gamma contribution was calculated following Prescott and Stephan (1982), and water concentration was assumed to be 5 ± 2 % by weight. Most samples indicated inhomogeneous bleaching, and therefore the mean of the least 30 % of the palaeodose values (4–6 aliquots) was used for age calculations (Srivastava et al. 2009; Ray and Srivastava 2010). This approach helped to identify and discard ages from aliquots that contained a population of poorly bleached quartz grains. Recently, a study indicated that that possibly due to high-energy sedimentary processes (deposition due to episodic flash floods, debris flow, etc.), modern sediment (zero age) from Spiti River yielded an OSL age of *2 ka. This implied that luminescence ages from this area might be overestimated due to poor bleaching (Anoop et al. 2012). Anoop et al. (2012) did not present any information on the sedimentology and the type of OSL analysis of this sample, and hence we were unable to use this for our studies. We took care in collecting the most suited samples for optical dating. The samples were collected from thin sand lenses embedded in gravel units or the lacustrine units. Sedimentation of thin sandy lenses even in gravel units represents waning phase of flood and does not represent a high-energy sedimentation process as stated by Anoop et al. (2012), and lakes in general represent a low-energy sedimentary regime. We surmise that such samples would have a finite time for bleaching.

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Table 1 Radioactive element concentrations, dose rates, palaeodose and ages of the samples collected from different sections and terrace deposits along the Spiti River Sample no.

Laboratory no.

Depth (m)

U (PPM)

Th (PPM)

K (%)

Palaeodose (Gy) Mean

Least

Dose rate (Gy/ka)

Age (ka) Mean

Least

Landslide zone (N 32° 020 13.300 E 78° 150 13.500 ) SP-1

LD-217

2.0

5.0

1.68

28 ± 10

27 ± 6

2.6 ± 0.2

11 ± 4

11 ± 3 8±1

Seko-Nasung section (N 32° 090 55.800 E 78° 070 07.000 ) SP-2

LD-218

12

0.6

1.5

1.33

18 ± 4

14 ± 2

1.7 ± 0.1

10 ± 3

SP-3

LD-219

6

1.8

4.1

1.48

15 ± 3

15 ± 1

2.4 ± 0.2

6±1

6±1

SP-4

LD-220

26

0.8

6.3

1.58

34 ± 11

20 ± 3

2.3 ± 0.2

15 ± 5

13 ± 2

Hansa section (N 32° 260 4800 E 77° 500 1700 ) SP-5

LD-221

12.5

1.0

6.5

1.97

40 ± 7

37 ± 3

2.7 ± 0.3

15 ± 3

14 ± 2

SP-6

LD-300

6

1.4

5.0

1.62

17 ± 3

17 ± 1

2.3 ± 0.2

8±2

7±1

SP-7

LD-301

0.7

2.3

5.7

1.73

24 ± 8

20 ± 4

2.7 ± 0.2

9±3

7±1

Kioto section (N 32° 260 14.500 E 77° 540 25.500 ) KyTL

Kioto

13

1.4

7.7

0.8

16 ± 2

16 ± 1

1.8 ± 0.1

10 ± 1

9±1

SP-8

LD-302

24

1.2

3.5

1.81

26 ± 4

23 ± 1

2.4 ± 0.2

11 ± 2

10 ± 1

4.3

1.56

66 ± 9

57 ± 6

2.1 ± 0.2

32 ± 4

28 ± 3

Pagmo-Hul section (N 32° 240 20.300 E 77° 560 11.100 ) SP-11

LD-305

48

0.6

Terraces at Kaza (N 32° 150 07.500 E 78° 020 11.7100 ) SP-12

LD-231

2.6

5.5

1.78

27 ± 10

22 ± 3

2.8 ± 0.2

10 ± 3

8±1

SP-13

LD-232

1.7

4.8

1.78

36 ± 2

25 ± 4

2.6 ± 0.2

14 ± 9

10 ± 2

SP-14

LD-233

3.1

8.1

2.4

38 ± 9

31 ± 6

3.7 ± 0.3

10 ± 2

9±2

0.3

2.6

0.86

23 ± 9

15 ± 2

1.3 ± 0.5

19 ± 8

12 ± 2

2.6

4.3

2.42

121 ± 22

102 ± 7

3.3 ± 0.3

37 ± 8

31 ± 4

2.8

5.4

2.73

127 ± 44

110 ± 15

3.7 ± 0.4

34 ± 12

30 ± 5

2.0

6.5

1.78

143 ± 10

140 ± 8

2.7 ± 0.2

53 ± 6

51 ± 5

1.6

5.8

1.57

128 ± 21

121 ± 7

2.4 ± 0.2

54 ± 10

51 ± 5

Lingti section (N 32° 060 30.900 E 78° 100 56.400 ) SP-15

LD-234

32

Sumdo section (N 32° 030 03.300 E 78° 360 21.400 ) SP-20

LD-212

30

Salkhal section (N 31° 530 27.000 E 78° 340 53.000 ) SP-21

LD-213

58

Hurling (N 32° 040 11.600 E 78° 350 50.800 ) SP-22

LD-214

Retti section (N 31° 570 18.200 E 78° 360 08.200 ) SP-23

LD-215

The average of the lowest 30 % palaeodoses was used in age estimation. Moisture content of 5 ± 5 % was assumed for all samples, and the cosmic ray Gamma contribution was calculated following Prescott and Stephan (1982)

Results

Well-sorted imbricated gravels

Lithofacies

This facies comprises 2- to 15-m-thick well-sorted clastsupported gravels. Clasts range from 2 to 15 cm in size, are well rounded and show imbrications. Internally, the lithofacies units are divided into several fining upward units (Fig. 5b). Laterally, the individual units are lensoidal, showing development of cross-beds with erosional bases. Often, lenses of cross-bedded fine sand separate depositional episodes. Alternating rippled fine sand and silt or poorly sorted matrix-supported gravel units in general overlie the facies. These arise from channelized flow deposited in form of a braid bar. The association between alternating rippled sand and silt suggests the development of channel levee, while that with poorly sorted matrixsupported gravel suggests channel disruption due to landslide activity (Srivastava et al. 2008; Ray and Srivastava 2010).

Parallel bedded gravel This facies comprised moderately sorted, weakly imbricated, parallel bedded gravel up to 10–40 m thick and is exposed towards the headwaters of the river at Losar and Hansa. Individual beds are internally fining upward with clast size ranging from 2 to 10 cm with smaller clasts being more angular (Fig. 5a). The matrix comprises coarse-grained sand, up to 5 % by volume. The unit includes thin lenses (10–25 cm) of rippled fine sand. This facies towards the downstream transforms into imbricated well-sorted gravel. The facies character suggested that it is a deposit of a glacier outwash plain where sedimentation took place under sheet flow during high flow conditions (Gustavson and Boothroyd 1987).

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Fig. 5 Lithofacies a parallel bedded gravel, b well-sorted imbricated gravels

Poorly sorted matrix-supported angular gravel Poorly sorted, matrix-supported angular gravels are 5–30 m thick. Clasts in this facies are disorganized and range from few cm to 1.5 m except at few places where it show the development of weak bedding. The individual units are devoid of internal physical structure. Thin units of fine sand often separate gravel units. Laterally, this lithofacies runs for several tens of meters and forms debris cones that are often vertically followed by parallel laminated clays and imbricated well-sorted gravels. These were a consequence of massive landslides or slope failure of debris cones that eventually dammed the river channel.

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Fig. 6 Lithofacies a parallel laminated clays, b cross-bedded sand

rippled or laminated fine sand. At places, it also shows iron reddening and moderate bioturbation. In Subfacies-a, light-coloured layers were deposited under higher-energy conditions when glacial melt water added sediment to a lake. During winter, when melt water and input of associated suspended sediments are reduced, and the lake surface freeze, dark laminae are formed. Subfacies-a indicates deposition in a lake with strong seasonality with temperatures reaching below freezing. Subfacies-b lacks varves but contains rhythmites in the form of silty and sandy layers, indicating longer summers and short winters, with temperatures uniformly above freezing point. The iron reddening and bioturbation suggests shallowing of the lake with sub-aerial exposure of the sediments. This is a warm and dry episode.

Parallel laminated clay Cross-bedded sand This facies is recognized into two subfacies. Subfacies-a comprise 0.5- to 1.0-m-thick units of 2- to 3-mm-thick varved clay laminae with light and dark colours. Subfaciesb show yellowish centimetre scale clayey laminations with no colour alternation (Fig. 6a). These are interlayered with

This 1.5- to 4.0-m-thick facies comprises fine to medium sand showing planar and trough cross-laminations (Fig. 6b). This facies usually overlies well-sorted imbricated gravel and is overlain by alternating rippled fine sand

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and silt lithofacies. The individual units are divisible into several fining upward depositional episodes that are lensoid in lateral geometry. This facies is prevalent in the sections at the confluence Parachu and Spiti rivers at Sumdo. These are the deposits of channel bars. The confluence played an important role in the formation of sandy channel bars in an environment dominated by gravel transport. At the confluence, the channel width and sediment load of the main channel of Spiti increase and this in turn reduced stream power of the channel. This condition promoted sediment partitioning and development of sand-dominated bars.


Lithofacies association C is made up of matrix-supported angular gravels, followed by parallel laminated clay and intermittent parallel laminated sand. This facies association overlies channel bound deposits and indicates formation of a lake due to channel blockage by landslide (Phartiyal et al. 2009a, b). A lateral litholog of *600 m near Seko-Nasung (described later) indicated landslideborne sediment at the base followed vertically by a laterally lensoid unit of parallel laminated clays, capped by channel deposits. Internally, the lacustrine deposits are composed of several shallowing cycles with parallel laminated sand marking a shallowing event.

Parallel laminated sand Stratigraphy of the sequences and luminescence dating It is 0.25- to 4-m-thick parallel laminated grey-coloured fine sand. The individual laminae are from mm to a cm thick that are often interbedded with ripple-laminated fine sand. It is found towards the top of well-sorted imbricated gravel and cross-bedded sand and is sometimes interbedded with matrix-supported angular gravel and parallel laminated clays. This facies forms a bar top when associated with wellsorted imbricated gravel and cross-bedded sand, which was deposited during receding flood conditions. It is associated with matrix-supported gravel, indicating sedimentation during the waning phase of landslide event, in small channels and rills that are often developed on the surface of a landslide cone. The facies when associated with parallel laminated clays indicate shallowing phase of lacustrine sedimentation. Lithofacies associations Based on genetic link, sedimentary facies can be grouped into three lithofacies associations, viz. A, B and C. Lithofacies association A is made up of thick units of parallel bedded gravels, interbedded with thin lenses of fine to medium sand. The sandy units may be parallel laminated or cross-bedded. These are deposits of glacial outwash. Lithofacies association B comprises well-sorted imbricate gravels, followed by cross-bedded sand and parallel laminated sand with few or no muddy units associated with it. At places, the sequence is interbedded with parallel bedded gravels. The thickness of this facies association sometimes exceeds 40 m and extends several kilometres laterally. Vertically, it is often consists of several fining upward depositional episodes. This indicates sedimentation in a channel on an accreting bar, and lower amounts of mud indicate that the channel was braided with high bed load. The banks were unstable, and sediment from the hill slopes was being easily eroded and transported to the channel.

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Twelve representative sections, depending upon the maximum lithofacies representation and accessibility, were chosen to understand the variation in sedimentation pattern and morphostratigraphy along the river valley (Fig. 1), and of these 10 sections were optically dated. The results of luminescence dating are listed in Table 1. In the following, we describe the landform, sedimentology, chronology and seismites within these sections from the origin of the river to its confluence with the river Sutlej. Losar section (N 32° 260 2800 E 77° 460 26.500 ) This *15-m-thick section, rises from the riverbed, is located near the headwaters (Fig. 7a). The basal 13 m is composed of lithofacies association A that is overlain by 2-m-thick unit of lithofacies association B. Moving *2 km downstream of Losar, the basal unit pinches out and merges into lithofacies association A. This suggests that the area was under the influence of paraglacial processes depositing sediments via sheet flows followed by braided channelized conditions. Hansa section (N 32° 260 4800 E 77° 500 1700 ) This section is located * 8 km downstream of Losar on the left bank of the river (Fig. 7b) where a 24-m-thick section rises from the riverbed (Fig. 7c). The stratigraphy of basal 14-m-thick deposits, similar to that exposed at Losar, comprises lithofacies association A. This is overlain by *10 m of lithofacies association B. Seismites are seen in the middle Hansa section. These are of simple morphology constituting of convolute and pinch and swell structures. The sample collected from the top of the basal unit yielded an age of 14 ± 2 ka (SP-5), and two samples from the middle and top of the overlying units gave ages of 7 ± 1 ka (SP-6 and SP-7). This suggests that until 14 ka


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Fig. 7 Sedimentary lithosection and chronology of sections at a Losar, b Hansa. c Field photograph showing 20-m-thick sequence rising from the riverbed at Hansa

glacial outwash plain was extended till Hansa and was subsequently taken over by fluvially dominated environment.

regime to braided glacial stream environment. The sample collected from the basal part of the section yielded an age of 28 ± 3 ka (SP-11).

Kioto section (N 32° 260 14.500 E 77° 540 25.500 )

Kaza section (N 32° 150 07.500 E 78° 020 11.7100 )

This section is located *4 km downstream of Hansa where basal *9 m is composed of lithofacies association A which is overlain by *20 m of lithofacies association C (Fig. 8a). Several levels of seismic structures are also noticed in the Kioto palaeolake section (Sangode and Mazari 2007). The lithofacies of the section indicates that a paraglacial sheet flow was interrupted by a landslide that gave way to a landslide-dammed lake sequence (Fig. 8b). Two samples collected from the top of the basal unit (SP-8) and the bottom of the lacustrine unit (KYTL) yielded ages of 10 ± 1 and 9 ± 1 ka, respectively.

This section is located *2 km upstream Kaza town. The section consists of terraces, viz. T1 located at 3.5 m and T2 at 11 m above the riverbed. A fan terrace FT3 overlies the terrace T2 (Fig. 10). Thus, morphostratigraphically, T-2 is the oldest, which is followed by the deposition of FT3 and T1. Terraces T1 and T2 are parallel to the Spiti River and are largely composed of several fining upward units of lithofacies association B, whereas FT3 was deposited by a tributary joining the Spiti from its right bank and is moderately sorted, stratified sub-angular gravels with lenses of cross-bedded fine sand. Three samples collected from T2, FT3 and T1 yielded ages of 10 ± 2 ka (SP-13), 9 ± 2 ka (SP-14) and 8 ± 1 ka (SP-12), respectively.

Pagmo-Hul section (N 32° 240 20.300 E 77° 560 11.100 ) The 50-m-thick section that lies on the right bank of the river is mainly composed of lithofacies association B (Fig. 9). The constituent gravels are mostly sub-rounded except for top *5 m of imbricated well-rounded gravels. This section is the result of braided channel conditions. The angularity of the gravels indicates the clasts are recycled from glacially deposited sediments and may represent a phase of high glacial discharge. This may indicate that, moving downstream, glacial sheet flows were channelised, indicating a transformation from paraglacial

Seko-Nasung section (N 32° 090 55.800 E 78° 070 07.000 ) This section is located *12 km downstream of Kaza. The section from the base is composed of 10- to 15-m-thick gravel and parallel laminated clays representing lithofacies association C followed by 10–12 m of well-sorted imbricated gravel of lithofacies association A. Good lateral exposure of this section provided an opportunity to understand the geometries of individual lithofacies associations. A lateral litholog of 600 m suggests that the basal

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Fig. 8 Kioto section a sedimentary lithosection with chronology, b photograph showing the lensoid lacustrine units and landslide cone at the base

Fig. 9 a Lithosection and chronology of Pagmo-Hul section. b Photograph showing cross-bedded angular gravels at Hul

matrix-supported debris resting on the riverbed is laterally extensive and runs for more than 600 m. The lithofacies forming a lacustrine sequence is lensoid with a maximum thickness of *15 and 480 m of lateral extent. The

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overlying fluvial gravels are also laterally extensive and are vertically divisible into several fining upward, lensoid, gravel bodies of lithofacies association A. Figure 11 shows the lateral litholog of this section. Two levels of simple


convolute structures, pinch and swell structures are seen in this section. The sequences at Seko-Nasung and Kioto also suggest that landslides that blocked the Spiti River to be more than 500 m in width. A sample collected from near the base of the basal unit yielded an age of 13 ± 2 ka (SP-4) and those collected from the base and towards the top of the overlying fluvial unit yielded ages of 8 ± 1 ka (SP-2) and 6 ± 1 ka (SP-3), respectively. This indicates the blockage of channel and the formation of a lake between 13 and 8 ka, and a fluvial regime between 8 and 6 ka. The sequence was incised by the river *6 ka. Lingti Section (N 32° 060 30.900 E 78° 100 56.400 ) This section occurs at a major geomorphic change as the shape of the valley abruptly changes from a broad U-shape to narrow V-shape, and this also shows a shift in terrace configuration, that is, relict fluvial deposits rest on a bedrock bench. This section is located at the left bank of Spiti

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River on Lingti-Lalung road. A 55-m-thick fluvial sequence resides on *25 m of exposed bedrock bench (Fig. 12a). From the base upwards, the sequence starts with *4-m-thick debris of lithofacies association C which is overlain by four cycles of fining upward units of imbricated gravels of lithofacies association B (Fig. 12a). The section includes strong deformation in the form of a folded fine sand unit penetrated by flame structures (Fig. 12b–d). The deformed units also show several faults of centimetre scale and flame structures along with complex convolutes, pseudonodules and pinch and swell structures. This sequence formed by fluvial aggradation and has preserved evidence of a strong seismic event. Such deformation structures may also form due to landslide close to a lake but the fact that the sequence does not show any lacustrine or landslide-related sedimentary association indicates that perhaps seismic activity was responsible for this deformation feature. A sample from the deformed unit *32 m below the surface yielded an age of 12 ± 2 ka (SP-15). This indicates that the aggradation started prior to 12 ka and the seismic activity occurred at *12 ka. Sumira Village section (N 32° 030 0000 E 78° 290 18.500 )

Fig. 10 a Field photograph showing river terraces T2, T1 and the fan terrace FT3 with the luminescence chronology. Note that morphostratigraphically the terraces T2 and T1 represent phase of one valley aggradation, whereas FT3 is a fan that sits on T2 and hence is younger than this terrace. b Schematic cross-section along X–Y showing the morphostratigraphy of the terraces and fan sequence

This section contains an epigenetic gorge and palaeovalley and it is located at the junction of a tributary. The palaeovalley, on the right bank of the Spiti River, is located at 9.5 m above the present river level. It is filled with several depositional cycles of matrix-supported angular gravels. The gorge is relatively narrow and is * 27 m deep as measured from the top of the rocky ledge present at the right margin of the palaeovalley (Fig. 13). The older channel of the Spiti River aggraded, shifted its course and incised to form the palaeovalley and epigenetic gorge. In the Alaknanda valley, such epigenetic gorges are mapped along the channel at several places (Pant 1975) and similar phases of aggradation have been documented (Srivastava et al. 2008). When a channel is laterally displaced as a result of either river blockage or rapid aggradation and incision occurs in bedrock rather than unconsolidated fills to base level, fossil valleys and

Fig. 11 Lateral litholog and luminescence dates of SekoNasung section. Note the lensoid nature of the lacustrine unit and that the aggradation in the Spiti River took place up to *6 ka

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Fig. 12 a Lithosection and luminescence chronology of the section at Lingti. Note the presence of a bedrock bench below the alluvial sequence. b Seismically deformed sandy unit, c flame structure, and d deformed laminae of the unit are enlarged parts (b)

Sumdo section (N 32° 030 03.300 E 78° 360 21.400 )

Fig. 13 Geomorphic configuration at Sumira. Note the presence of the sediment filled palaeovalley and the epigenetic gorge

epigenetic gorges are formed (Ouimet et al. 2008). Therefore, the sedimentary fill preserved at Sumira, and the epigenetic incision represents a phase of rapid filling of the valley. Unfortunately, we could not get any datable material from the section.

There are two river terraces at Sumdo, namely T2 and T1 (Fig. 14a). The terraces consist of bedrock benches with overlying alluvial cover. The upper terrace T2 lies *130 m above the present river level with the top 30 m being alluvial cover. The lower terrace T1 lies at *45 m above the river bed with *35 m of alluvial cover. The alluvial covers are composed of several cycles of fining upward imbricated gravels of lithofacies association B (Fig. 14b). This section shows two phases of alluviation separated bedrock incision. The river incised further from T1 to T0 following a second phase bedrock incision. The sample dated from near the bottom of T1 yielded an age of 31 ± 4 ka (SP-20). The sample collected from T2 showed luminescence saturation, but morphostratigraphically it is older than 31 ka. Salkhal section (N 31° 530 27.000 E 78° 340 53.000 )

Hurling section (N 32° 040 11.600 E 78° 350 50.800 ) The basal * 5 m is unexposed, and the exposed part of the sequence consists of * 35 m of poorly sorted matrixsupported gravels and parallel laminated clays of lithofacies association C. The topmost 1.5 m is made up of matrix-supported sub-angular gravels. The section is the result of a landslide-dammed lake on the Spiti River. A sample below the 3 m from the top surface of the sequence yielded an age of 51 ± 5 ka (SP-22).

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A *90-m-thick sedimentary sequence rests on 10 m of exposed bedrock bench. The sequence starts with *40 m of channel sediments of lithofacies association B, which is divisible into several fining upward depositional episodes. This is followed by *50 m of landslide-dammed lake sediments of lithofacies association C (Fig. 15). The section is the result of landslide damming of the Spiti River after 30 ± 5 ka (SP-21). The uplift and bedrock incision occurred later.


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Fig. 14 a Photograph showing the terraces at Sumdo. Note the presence of a bedrock bench below the two levels of terraces. b Schematic configuration of the terraces, with lithosections of the alluvial cover of the two terraces with chronology

Fig. 15 a Lithosection of the sedimentary sequence at Salkhal. Note the presence of bedrock below the alluvial sequence, with luminescence chronology. b Photograph showing the section at Salkhal

Retti section (N 31° 570 18.200 E 78° 360 08.200 ) This section contains two levels of river terraces, namely T2 (upper) and T1 (lower). T2 and T1 are *150 and *66 m

above the riverbed, respectively. The 33 m of cover of T2 is composed at the base of 30 m of landslide-dammed lacustrine sediments of lithofacies association C, while the top 3 m is made up of imbricated gravels of lithofacies

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association B. T1 consists of a basal 3 m of channel sediments of lithofacies association B followed by 13 m of lacustrine sediments representing lithofacies association C. T2 preserves an episode of landslide damming. OSL date from the top of the sequence indicates this landslide and subsequent damming took place at 51 ± 5 ka (LD-215). A similar event was also dated at Hurling at 51 ± 5 ka (LD215).

Discussions Style of valley aggradation in an arid region of Himalaya The sedimentary architecture of cliff sections preserved along *150 km stretch of Spiti River, from the headwaters to its confluence with the Sutlej River, was examined. In the headwaters of the Spiti valley, lithofacies composition of sequences from Losar to Hul indicates paraglacial processes in the form of sheet flow deposition followed by episodes of landslide and channel damming. Luminescence dating at Hul, Hansa and Kioto suggests that paraglacial sedimentation started *28 ka (basal age at Hul sequence) and continued up to 14 ka (basal age at Hansa, Fig. 7b). Upper part of landslide sediment at Kioto was dated to 10 ka and the lake sediments 9 and 7 ka at Hansa (Figs. 7b, 8). This implies that valley aggradation started due to increased sediment supply from glacial outwash between 28 and 14 ka, with channel damming and formation of a lake between 14 and 7 ka. Further downstream, at Kaza, sedimentation dominated by channel processes took place between 10 and 8 ka with tributary channels carrying high sediment loads forming fans. The progradation of tributary fans into the main channels implies that the hydraulic competence of the main channel, as compared to the present day, was low. Similarly, landsliding and lake formation at Seko-Nasung took place between 13 and 7 ka, and the sequence at Lingti was also deposited around the same time (12 ka, Fig. 12). Sequences at Hurling, Sumdo, Salkhal and Retti all rest on bedrock benches and were deposited from [30 to \50 ka, all of which involved landslide-dammed lacustrine aggradation. Thus, extensive mass wasting and channel damming facilitated aggradation in the entire valley during 14–6 ka in the upper valley and at [50–30 ka in the lower valley. Rivers in the Himalaya aggrade and incise in response to (1) climate-related factors such as discharge, sediment load, vegetation cover and (2) tectonic perturbations such as local uplift and the subsequent formation of intermontane basins. However, the response time of a river to global climatic changes and the style of aggradation depend upon climatic and orographic domains through which it drains

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(Blum and To¨rnqvist, 2000). Spiti River lies in rain the shadow zone of Himalaya with *50 mm rainfall annually and temperatures remaining below freezing during most part of year. During abnormal monsoon years, when the valley receives unusually high rainfall (*1,000 mm; see Bookhagen et al. 2005a, b), massive landslides block the main channel and form landslide-dammed lakes. Such episodes produce a sedimentary profile, exhibiting a channel deposit at the base followed by landslide-borne gravels and fine-grained lacustrine sediments (Phartiyal et al. 2009a, b). Trans-Himalayan rivers, such as river Spiti, have lower discharge, higher sediment to water ratio and lower stream power than their counterparts in wetter Himalaya, allowing landslide-dammed lakes to last longer and their sedimentary sequences to be preserved. It has been reported that similar landslide-dammed lake sustained for several thousand years in moderately humid Baspa valley also (Bookhagen et al. 2006). Glacial, landslide and fluvial processes are the geomorphic agents that sculpt mountains. A review of palaeogeomorphic changes in the trans-Himalayan region indicates a higher frequency of landslides and landslidedammed lakes soon after the last glacial phase (Sundriyal et al. 2007; Dortch et al. 2009). Dortch and others using cosmogenic radionuclide dated several palaeolandslides in the NW Himalaya (see Dortch et al. 2009, 2011) including Lahul and Spiti valley and inferred that these events occurred between 15 and 5 ka. This age range of 15–5 ka landslides occurrences correlated well with period of postLGM increased monsoon (see Prell and Kutzbach 1987; Bookhagen et al. 2006). In the present work, a sand lens from a massive landslide downstream of Lingti is also dated to 11 ± 3 ka (SP1; Table 1) and belongs to the same time frame. Figure 16 shows published chronology of landslide and synchronous aggradation in the Spiti River valley (Dortch et al. 2009). Similar inferences drawn from chronological records of landslides in the Italian Dolomites (Soldati et al. 2004) and from the Argentine Andes (Trauth et al. 2003) suggest increased landslide and channel damming in response to high rainfall. Chronological data for glacial stratigraphy in monsoon-affected regions of Himalaya show that glaciers expanded to their maximum extent during Marine Isotope Stage III (MIS III). In the western Himalaya, cosmogenic radionuclide dating of moraines in the Lahul valley (adjacent to and west of Spiti) suggests glacial advance (the Batal Glacial Stage) at 15.5–12 ka and again at 11.4–10 ka (Kulti glacial stage; Owen et al. 2001). Chronologies of moraines of Pin and Thangi valleys indicated maximum glaciation at *18 ka (Scherler et al. 2010). The chronology of landslide-driven aggradation of Spiti River, presented here, indicates (i) extensive mass wasting during the intensified monsoon after last glacial phases of MIS-II and


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Fig. 16 Chronology of landslides and aggradation in the Spiti River valley, showing a role for increased SW Monsoon (Modified after Dortch et al. 2009). Phase I occurred at 50–30 ka and Phase II at 15–6 ka during warm phases

MIS-III and (ii) the basal part of the sequence from Losar to Hul suggests advance of glaciers in the valley head at 28–14 ka. Thus, the aggradation history of the Spiti valley suggests that valley filling by mass wasting occurred during an intensified monsoon and the incision took place during middle Holocene when sediment to water ratio decreased. Sediment in such river systems is largely supplied from the hill slope failures, tributary fans and moraines, throughout the valley. During drier conditions, in rivers of wetter Himalaya, for example, Alaknanda, only upper and middle part of the catchment become arid–semiarid and the lower part where aggradation occurs remains wet and vegetated. Thus, during the transition from a dry to wetter climate, sediment is largely supplied from the upper–middle parts of the valley and deposited in its lower reaches. Therefore, most aggradation in Alaknanda occurred in form of channel-borne aggradation (Ray and Srivastava 2010). This glacial–paraglacial hypothesis enables landform connectivity in a glacially sourced river and suggests that lower part of the valley alleviates and derives sediments from the deglaciated part of the upper valley mostly during warmer interludes (Church and Slaymaker 1989). Geomorphic evolution of the Spiti River valley since the last glacial phase The geomorphic evolution of the Spiti River valley is determined by three factors, viz. (1) glacial action, (2) mass wasting and (3) tectonics in trans-Himalayan. The present study suggests a three-stage evolution of Spiti valley during the past 50 ka.

Stage I (28–14 ka): During the last glacial phase of weak monsoon, glaciers expanded in the Spiti valley (suggested by glacial outwash gravel up to Kioto), and river discharge was controlled by glacial melting. This resulted in a braided river system, and the sediment transport and deposition were controlled by paraglacial processes. The slopes were barren and frigid, leading to marked physical weathering and debris cone formation. Stage II (14–8 ka): In this phase of monsoon intensification, the Spiti River valley experienced periods of high rainfall. Debris cones cascaded into the main channel as debris flows and unstable hill slopes generated massive landslides causing damming and formation of numerous small lakes throughout the length of the river (Bookhagen et al. 2006). These lakes lasted typically for 3–5 ka. Wetter conditions after 8 ka increased vegetation and stabilized both the slopes and debris fans. Such a condition reduced sediment load in the channel and increased stream power. This in turn resulted in increased sediment removal and valley incision. Incision of lake sediments took place *6 ka as is evident from the Seko-Nasung section. Such conditions prevailed along the valley in the region upstream of Hurling. Downstream from Hurling, Sumdo, Salkhal and Retti, the sequences suggest similar palaeogeomorphic and climatic conditions during [30–*50 ka. This corresponds to an older episode of warming and intensified monsoon that preceded the MIS III/IV. Rainfall at the termination of glacial phases and initiation of wet conditions is normally episodic when monsoon front competes with the cold fronts and a local build-up of rain systems takes place (Li and Fu 2004). Such phases generate

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landslides and slope failures. We envisage such a climatic set-up in Spiti valley during 14–8 ka. Stage III (\7 ka): The upper part of the valley to SekoNasung is wide, where the channel is braided and the relict fluvial deposit rises from the present day riverbed. However, from Lingti downstream, the channel is narrow, meandering, and all the sedimentary sequences are located above the channel on bedrock benches. This implies that this valley responded to tectonic deformation along a normal fault where the valley upstream of Lingti is a downthrown block. Based on the youngest aggradation date derived from Seko-Nasung, we place this fault activity \6 ka and infer a fault just upstream Lingti at Mane. Recent studies on sedimentary profile from a palaeolake at Mane suggest four episodes of tectonic activity along this fault and that the lake was formed in response to early Holocene warming and increased rainfall (Anoop et al. 2012). The transitions between geomorphic segments in lower and upper valley occur between Seko-Nasung and Lingti villages. Changes in channel planform from braided to meandering, incised river bed and the occurrence of uplifted terraces, in the meandering stretch, from downstream of Lingti indicate the presence of a fault. We also report seismites near Lingti optically dated to *7 ka. Thus, the study indicates that this fault was reactivated at *7 ka. Structural observations in the vicinity (Lingti valley) by Neumayer et al. (2004) suggest the presence of bookshelf-type extensional faults or steeply dipping conjugate brittle normal faults. This is in accordance with instrumentally monitored seismicity in this region (see Molnar and Chen 1983). Such faults are associated with faulting of lake sequences and soft sediments deformation in Lingti valley and adjoining area (Neumayer et al. 2004) as described previously (Mohindra and Bagati 1996; Baneerjee et al. 1997; Dubey and Bhakuni 2004; Singh and Jain 2007). It is suggested that such brittle deformation can occur in the hanging wall extension of KCnf. Several other such faults have been well documented from the transHimalayan region (Thiede et al. 2006; Hintersberger et al. 2010). These faults indicate E–W-oriented crustal extension, contemporaneous to N–S shortening in the Himalaya. This, however, needs further confirmation. In summary, the present study indicates that during Early-Mid Holocene, Spiti River valley experienced deglaciation, increased precipitation and seismic activity along the KCnf. This necessitates an explanation as to whether active land sliding was facilitated just by increased rainfall or glacial retreat and subsequent debuttressing of slopes and/or seismic shaking along KCnf acted as additional forces. In the Alpine SE France, rock slope failures concentrated within the area that was ice-covered during LGM. This

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suggested glacial debuttressing to be a controlling factor (Cossart et al. 2008). In our view, glacial debuttressing and enhanced precipitation after the LGM might have triggered the landslides. The role of KCnf may at present be able to be discounted because the locations of the major landslides in the area do not follow the fault trend.

Conclusions Following conclusions can be drawn from this study: 1.

2.

3. 4.

The Spiti River valley can be divided into two segments: (i) upper valley with broad, braided channel where relict sedimentary sequences rise from the riverbed; (ii) lower valley that is narrow, meandering, incised into the bedrock, and here, the relict fluviolacustrine deposits sit on the bedrock bench located above the riverbed. The transition of these geomorphic changes lies between Seko-Nasung and Lingti at Mane. Sedimentary sequences have three lithofacies associations: (A) representing glacial outwash; (B) indicative of sedimentation in an accreting bar under braided conditions; and (C) representing formation of lakes due to channel blockage by landsliding. The valley is usually arid, so any period of mass wasting-related channel damming and lake formation is likely during intensified monsoon phases (Bookhagen et al. 2005a, b; Phartiyal et al. 2009a, b) Luminescence dating indicated two phases of intensified monsoon during at 14–8 and 50–30 ka. Between Seko-Nasung and Lingti at Mane, a fault is inferred, which developed broad and braided upper valley and narrow and meandering lower valley. Luminescence dating suggests that this fault was active *7 ka.

Acknowledgments We thank the Directors of the Wadia Institute of Himalayan Geology, Dehradun, and the Birbal Sahni Institute of Palaeobotany, Lucknow, for encouragement. Prof. A.K. Singhvi and Prof. R.J. Wasson are acknowledged for pre-submission reviews on the manuscript. Bodo Bookhagen and two anonymous reviewers helped improve the manuscript. Drs. Rasmus Theide, Barun Mukherji, S. Mukherjee topic editors, is thanked for his efforts in bringing up this contribution.

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