Morpho-sedimentary characteristics of the Quaternary Matiali fan and associated river terraces, Jalpaiguri, India: Implications for climatic controls

GEOMOR-04787; No of Pages 16 Geomorphology xxx (2014) xxx–xxx

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Morpho-sedimentary characteristics of the Quaternary Matiali fan and associated river terraces, Jalpaiguri, India: Implications for climatic controls Rimpal Kar a, Tapan Chakraborty a,⁎, Chandan Chakraborty a, Parthsarathi Ghosh a, Anil K. Tyagi b,c, Ashok K. Singhvi b a b c

Geological Studies Unit, Indian Statistical Institute, Kolkata 700108, India, Physical Research Laboratory, Ahmedabad 380009, India, ITER, Institute for Plasma Research, Bhat, Gandhinagar, Gujarat, India

a r t i c l e

i n f o

Article history: Received 14 April 2013 Received in revised form 24 April 2014 Accepted 19 May 2014 Available online xxxx Keywords: Alluvial fans River terraces Eastern Himalaya Tectonics vs climate Fault-propagation folds

a b s t r a c t The Matiali fan is a coarse-grained, small alluvial fan in the eastern Himalayan foothills. It co-exists side by side with the large Tista megafan and other Quaternary fluvial deposits, and has been affected by a number of young thrust faults. It is generally believed that tectonics is the main control in the deposition of these proximal fan–terrace systems. In this paper, geomorphologic and sedimentologic study of the Matiali fan and associated river terraces are combined with five OSL dates from these deposits to understand the succession of events and the forcing mechanism that shaped the geomorphology in the study area during late Quaternary time. Two aggradational terraces (T1 and T2; T2 N T1) occur within the river valleys incised on the Matiali fan. Three E–W scarps cross the fan surface, and they represent the steeper limb of the asymmetric fault-propagation folds formed over blind thrusts. These folds have deformed the fan (T3) and T2 terrace sediments, but the youngest T1 terrace deposits have remained undeformed. Sedimentological studies indicate continuous gradation from the coarsening-upwards mass-flow megagravel in the proximal part to the traction transported finer sheetflood gravels in the distal part, implying a continuous sedimentation history across the fan, uninterrupted by any evidence of syn-depositional tectonic movement. Poorly consolidated sandy gravels of the terraces indicate deposition through braided fluvial processes during a later period of sediment aggradation that filled up the incised river valleys. Previously published 14C dates indicate that deposition of the Matiali fan started around 34 ka coinciding with a period of the intensified Indian summer monsoon of MIS-3. It is suggested that the fan was abandoned and river valleys incised during the LGM between 24 and 18 ka when the discharge decreased substantially. Increased rainfall and sediment supply, with their inherent fluctuations, during wetter periods of MIS-2 and MIS-1 since 12 ka probably resulted in the aggradation of T2 and T1 as shown by our OSL dates. OSL dates from the top of deformed T2 and base of undeformed T1 indicate that the Chalsa fold formed between ~ 11 and ~ 6 ka. Succession of geomorphic and deformational events reconstructed from this study and available age data indicate that the Matiali fan and terrace aggradation coincides with periods of increased monsoonal precipitation, whereas tectonic movements along blind thrusts of Chalsa and Matiali took place later, deforming the fan and older terrace deposits. The evidence unequivocally indicates, contrary to the prevalent notion of tectonic control of geomorphic features in the proximal mountain-front setting, that the deposition of the fan–terrace system was primarily controlled by the fluctuation of the Asian summer monsoon rather than Himalayan tectonics. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The Himalayan mountain belt is the largest active continent– continent collisional orogen initiated by the collision of the Indian and Tibetan plates in Middle Eocene (~ 50 Ma). A number of north-dipping, southward extruding, thrusts (i.e., Main Central ⁎ Corresponding author. Tel.: +91 33 2575 3150 (office), +91 33 2429 2451 (home); fax: +91 33 2577 3026. E-mail address: [email protected] (T. Chakraborty).

Thrust, Main Boundary Thrust etc.) advanced progressively towards the Indian craton with time. The peripheral foreland basins formed in response to flexural loading of the lithosphere and migrated cratonward with the southward propagation of the fold–thrust belt (Yin, 2006). The Indus–Ganga–Brahamaputra basin, the modern foreland, probably formed about 2 Ma ago with the onset of the Main Frontal Thrust (MFT) (Wesnousky et al., 1999). MFT, the southernmost expression in the Himalayan thrust system (Gansser, 1964), transports the Neogene and earlier strata over the Quaternary deposits of the Indo-Gangetic foreland basin.

http://dx.doi.org/10.1016/j.geomorph.2014.05.014 0169-555X/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: Kar, R., et al., Morpho-sedimentary characteristics of the Quaternary Matiali fan and associated river terraces, Jalpaiguri, India: Implications for climatic controls, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.05.014

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R. Kar et al. / Geomorphology xxx (2014) xxx–xxx

Numerous Quaternary mountain-attached alluvial fans occur all along the foothills of the Himalaya. In eastern Himalaya they occur side by side with large low-gradient megafans and other Quaternary fluvial deposits (Singh, 2004; Tandon et al., 2008; Chakraborty, 2010). The alluvial fans are dissected by the modern rivers emanating from the mountain belt and are affected by neotectonic faults (Nakata, 1989; Malik and Nakata, 2003; Kumar et al., 2006, 2010; Singh and Tandon, 2010). The proximal foreland throughout the Himalayan foreland is considered tectonically active and many of the Quaternary fan and terraces of the frontal Himalaya have primarily been attributed to the ongoing tectonic movements (Singh et al., 2001; Chakrabarti Goswami et al., 2013). However, globally there is a debate in alluvial fan literature on the tectonics vs climate question (Viseras et al., 2003; Harvey et al., 2005; Leeder, 2011). Within this debate, and particularly in the case of the Quaternary deposits, consensus appears to be that deposition and incision of fan–terrace systems are primarily controlled by climatic forcing (Pratt et al., 2002; Bookhagen et al., 2006; Pan et al., 2007; Jones et al., 2014). This work examines one such mountain attached fan, the Matiali fan in the Neora–Jaldhaka Valley of the Jalpaiguri District, West Bengal, and attempts to evaluate the relative role of climate and on-going tectonics in the development of the fan and the associated river terraces. We present data on the fan morphology, morphometry of the associated catchment basins, terrace stratigraphy, sedimentology of the fan and terrace deposits and deformations affecting these sediments. Five new OSL dates combined with the published 14C dates provide a broad chronologic framework of the sequence of tectono-geomorphic events that has affected the Quaternary

sediments in this part of the foreland. This paper presents evidence that suggest that the development of the mountain-front fan and the associated terraces are principally controlled by fluctuation in the monsoon strength rather than the tectonic activity of the Himalayan mountain belt. 2. Geological setting The Indus–Tsangpo suture in the north and the Main Frontal Thrust (MFT) in the south delineate the Himalayan orogen. From north to the south, major thrusts divide the Himalayan fold–thrust belt into three distinct orographic domains, viz., the Higher Himalayas, the Lesser Himalayas and the Sub-Himalayas. The southern end of the Himalayan fold–thrust belt is inferred to be tectonically active with the presence of active faults (Nakata, 1989; Powers et al., 1998; Malik and Nakata, 2003; Mukhopadhyay and Mishra, 2004; Kumar et al., 2006; Guha et al., 2007; Kumar et al., 2010; Singh and Tandon, 2010). The eastern Himalayan mountain-front, in contrast to the western Himalaya, is characterised by the occurrence of a number of megafans alongside the smaller, coarse-grained, mountain-attached alluvial fans and river terrace deposits. The origin of these megafans has been debated invoking tectonic (Gupta, 1997) or climatic (Burbank, 1992) factors or a combination of the two (Sinha et al., 2005). The present study area is part of the foothills of Darjeeling Sikkim Himalaya, in the Jalpaiguri District of West Bengal (Fig. 1). Three abandoned Quaternary alluvial fans documented between Chel River in the west and Ghatia River in the east are the Rangamati fan, Matiali fan, and Nagrakata fan (Nakata, 1989).

Fig. 1. a) Geomorphologic map of the Matiali fan. Major geomorphic surfaces, terraces, and thrusts as inferred by earlier workers are shown. The inset diagram shows the location of the study area in West Bengal, India. b) Outline of the fan and major stream courses in the study area. c) A SRTM DEM visualisation of the Matiali fan. Major geomorphic surfaces and lineaments/scarps are marked. Panel (a) is modified after Nakata (1989).

Please cite this article as: Kar, R., et al., Morpho-sedimentary characteristics of the Quaternary Matiali fan and associated river terraces, Jalpaiguri, India: Implications for climatic controls, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.05.014

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Based on the analysis of geomorphic features, Nakata (1989) identified several active faults and recognised three geomorphic surfaces in the piedmont deposits, viz., Matiali, Rangamati and Samsing (in order of younging) and two river terraces in the area (HRT: higher river terrace and MRT: middle river terrace) (Fig. 1). However, later workers (Das

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and Chattopadhyay, 1993a,b; Guha et al., 2007) have questioned the interpretation of the chronology of these surfaces, and based on their observation of the soil profiles mantling these surfaces opined that Matiali and Rangamati surfaces belong to the same fan deposits and Samsing is the oldest surface being capped by an oxidised red soil horizon.

Fig. 2. DEM visualisation of the Matiali fan and adjacent area. Note the low-gradient sediment lobes occurring south of the Chalsa lineament. Yellow circles show the sample locations for OSL dating. The yellow star is the sample location of 14C dating by Guha et al. (2007) (Sp1: 33, 875 ± 550 Ybp). White rectangle (K1): location of trench site (Kumar et al., 2010). The figure defines the Lmf and Ls (green lines) used for calculating mountain front sinuosity. E–W and N–S dotted pink lines, marked (a) and (b) respectively, are section lines for the fan shown in Fig. 2(a) and (b). Sections lines are drawn using SRTM DEM data with 90 m ground resolution. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Please cite this article as: Kar, R., et al., Morpho-sedimentary characteristics of the Quaternary Matiali fan and associated river terraces, Jalpaiguri, India: Implications for climatic controls, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.05.014

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3. Geomorphology of the Matiali fan 3.1. General morphometry The triangular mountain-attached Quaternary sediment body near Samsing, bounded by Neora and Murti Rivers on its west and east respectively, is referred to as the Matiali fan (Figs. 1, 2). The fan is now abandoned and modern drainages have incised this fan body. The Matiali fan is about 10 km wide and it extends longitudinally for about 14 km. The maximum longitudinal surface slope is about 4° near the apex and that near the toe it is approximately 0.4°. The maximum cross-fan slope is 1.04°. The decrease in the longitudinal slope downfan produces the concave upward shape of the Matiali fan (Table 1; Fig. 2b). Within the study area the Neora, Kurti and Murti Rivers vary in length from 14 to 20 km and have an average slope of 1.33°, 1.24° and 1.31° respectively (Fig. 3; Table 1). Hypsometric analysis of the Murti catchment yields a hypsometric integral (Hi) value of 0.429. Hypsometric curves of the 1st and 2nd order streams in the Murti and Neora catchments show both convex upward and sigmoidal shapes, while the 3rd order streams are flatter in both the catchments (Fig. 4). Some of the 2nd order streams of the Neora catchment are pronouncedly convexed upwards. The Neora catchment basin yields a slightly higher hypsometric integral (Hi) value of 0.483 (Fig. 4; Table 1). The Mountain Front Sinuosity Index (Smf, c.f., Bull, 1977) yields a value of about 1.54 (Fig. 2).

3.2. Major geomorphic features Three prominent lineaments clearly visible in remotely sensed images are three prominent scarps traversing the Matiali fan surface. From north to south these are the Samsing, Matiali and Chalsa scarps. The Matiali fan surface is designated here as the T3 surface and is subdivided informally into T3a, T3b and T3c to indicate positions of the fan surface north of Samsing, Matiali and Chalsa scarps respectively.

The river valleys flanking the fan have two major aggradational terraces T1 and T2, of which T2 is older (Figs. 2, 5). The gentle southward sloping fan surface (average slope ≈ 2°) is broken by three scarps that slope around 17° to 9° to the south and southeast, resulting in remarkable changes in the elevation of the fan surface across these lineaments. Immediately north of the Chalsa and Matiali scarps the fan surface shows a very gentle northward slope, varying between 0.68° and 1.56° (measured from 90 m SRTM DEM using Global Mapper™ software; Table 1, Figs. 2b, 5). General subhorizontal to low southward dips of the fan strata change abruptly near Matiali and Chalsa scarp. Gravelly strata underlying Matiali scarp dip 45° to the south (Fig. 6) and stratification of sandy gravels of the T2 terrace exposed below Chalsa scarp, dip about 14° towards the south (Fig. 7). A similar scarp is present near Samsing but due to a lack of suitable exposures no data on the details of the surface geometry and internal stratification are available. The Matiali fan surface (mainly T3b and T3c) is dissected by numerous small gullies as is evident in the satellite images, SRTM DEM visualisations and in the Survey of India toposheet. These first order streams show a radial pattern on the T3b surface (Fig. 8). The density of these small, radiating channels is higher north of the Matiali scarp than that on the south of the scarp and a discontinuity in their pattern across the scarp is clearly visible in the western part of the fan surface (Fig. 8). Many of the first order streams north of the scarp coalesce to form larger streams before they cross the Matiali scarp. Many of the 2nd order channels show sharp bends to the east or west as they cross the Matiali scarp (e.g. the Juranthi and Kurti Rivers: Fig. 8). The Matiali scarp becomes subdued to the eastern part of the fan and the fan surface has a continuous slope to the southeast with radiating 1st order streams on the surface continuing unhindered to the T3b surface. Convergence of lower order channels and sharp bends in the course of some channels are also observed at the Chalsa scarp (Fig. 8). The radial pattern of small channels is nearly absent on the T3c surface, where several 2nd order streams flow sub-parallel to each other. In the eastern margin of the fan, a second order tributary stream west of Murti River, flows along a

Table 1 Geomorphic features of the Matiali fan, the surfaces on the fan, the scarps and rivers and their catchments of the area. Geomorphic features

Name

Character

Fan

Matiali fan

Surfaces

T3a (Samsing surface) T3b (Cheloni surface)

The Matiali fan: width — 10 km, longitudinal extent — 14 km, radius ~14 km. Maximum cross-fan slope in the upper reach 1.04° (R2 = 0.9925), average longitudinal slope of the fan surface is 2.08° (R2 = 0.9874) Maximum longitudinal slope at the fan apex is 4.05° (R2 = 0.9942) and minimum slope of the fan toe is 0.41° (R2 = 0.9594) Mountain Front Sinuosity Index (Smf = Lmf/Ls; see Fig. 2) = 1.53 T3a (Samsing surface) is the highest region of the Matiali fan. Average slope: 3.32° to south Average slope 2.09° southward; immediately north of Matiali scarp it dips northward by 0.68° to 1.56° (Fig. 6) Average slope 0.85° southward; immediately north of Chalsa scarp it dips gently northward by 0.94° to 1.56° (Fig. 6) •The maximum dip of the Samsing scarp is 12.80° (R2 = 0.9993), the Matiali scarp, 18.22° (R2 = 0.9997) and the Chalsa scarp, 10.51° (R2 = 0.9995) towards south to southeast

T3c (Ibil surface) Scarps

River

(Samsing scarp) (Matiali scarp) Chalsa scarp Neora

Murti

Kurti

•Left bank tributary of Tista River •Small tributaries within the mountain are, Dhaola Khola, Neora Nadi and Thosam Chu and Santola Khola. Catchment area NW of the Matiali fan •From the apex of the Matiali fan to the south of the Chalsa scarp: length — more than 16 km, average slope — 1.33° (R2 = 0.9981) (Fig. 3) •Catchment of the Neora River: area is 83.5 km2, perimeter: 49.3 km. Hypsometric integral (Hi): ~0.483 with a convex up and sigmoid hypsometric curve for lower order streams (Fig. 4) •The Murti Nadi originates near the MO Reserved Forest •Murti River in its upstream part deeply incises into the basement rocks of the catchment basin and the fan sediments •From the apex of the Matiali fan to the south of the Chalsa scarp: length ~20 km, average slope 1.29° (R2 = 0.9949) (Fig. 3) •Catchment of the Murti River: area — 41 km2, perimeter — 33 km, hypsometric integral (Hi): ~0.429 with convex up and sigmoid hypsometric curve (Fig. 4) •Kurti River originates at the upper part of the Matiali fan and it drains the surface water of the fan surface •Kurti Nadi is ~14 km long and with an average slope 1.02° (R2 = 0.9854) (Fig. 3)⁎⁎

⁎⁎ All slope related data obtained from SRTM 90 m DEM and R2 values were calculated by fitting a 3rd order polynomial curve through the elevation data obtained.

Please cite this article as: Kar, R., et al., Morpho-sedimentary characteristics of the Quaternary Matiali fan and associated river terraces, Jalpaiguri, India: Implications for climatic controls, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.05.014

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Fig. 3. Profile of three main rivers in the study area. Drawn from SRTM data.

straight line to the SSW direction (Figs. 2, 8). The Murti River, after the confluence with this second order tributary, also flows along the same straight course towards SSW. Similarly, the Neora River appears to follow a rather straight course to the south in the southern part of the study area (Fig. 8). Two major aggradational terraces, T1 and T2, and a few minor discontinuous terraces occur within the incised valleys of the Neora, Kurti and Murti Rivers (Figs. 5, 9). In the river valleys incised within or at the margin of the fan deposits, the fan surface forms the degradational terrace T3. A thin horizontal gravel layer with an erosional base mantles the dipping T2 strata and identified as discontinuous terrace T2a, that is preserved only in the west bank of the Kurti River near the Chalsa scarp (Figs. 7, 9). T1 and T2 are paired terraces with similar heights on both bank of the rivers and are confined only within the river valleys south of the Matiali scarp (Fig. 5). The elevation of T1, the lowest terrace, varies between 5 and 10 m above the modern river bed and the maximum elevation of T3c terrace near the fan apex is ~ 200 m above the river bed. 3.3. Geomorphic and sedimentary processes The triangular shape of the Matiali sediment wedge with a convexup cross profile, concave-up long profile, high surface slope, mass-flow dominated coarse sediments and a coarsening upward trend (described later in the Sedimentology section) collectively indicates that this wedge formed as a small alluvial fan (Bull, 1977; Blair and McPherson, 1994). The Murti and Neora Rivers deeply incise the basement rocks of the Daling Group north of the fan and incise the proximal fan sediments. Further downstream, these rivers are deflected by the preexisting fan sediment body, and move along the eastern and western margin of the fan. The incised valley of the Kurti River is confined within

the fan area. The nature of deflection of the rivers and incision of the fan sediments suggest that the present course of the rivers developed after the abandonment of the Matiali fan, and is not related to the paleochannels that once deposited the fan (cf. Goswami et al., 2012 for an interpretation to the contrary). Similarly, numerous first order streams on T3 are recent gullies incising the exposed surface of the fan. The size of these small streams clearly rules out the possibility of their depositing megagravels that comprise the Matiali fan (described in detail later), and is, therefore, unrelated to the coarse-gravel transporting fan paleochannels. The radiating pattern in these 1st order streams reflects the passive incision of the now inactive, convex upward surface of the Matiali fan by modern surface processes and is unrelated to the paleochannels that constructed the fan. The lower density of the 1st order channels on the T3c surface is probably a combined result of the discontinuation of many first order streams at the Matiali scarp, the gentler gradient of the T3c surface and the shorter length of time available to the surface processes for gullying. The T3c surface attained the present configuration and was exposed to erosional modifications after the formation of the Chalsa scarp. Since the density of the gullies is related to the time available to the surface processes to create the gullies, their lower density implies a younger age of the T3c surface as compared to that of T3b. This in turn, would imply a younger age of the Chalsa scarp that formed after the Matiali scarp. The available structural models that infer a progressive southward propagation of the Himalayan thrust front (Hodges, 2000), also support this interpretation. The straight courses of the smaller second order channels in the eastern part of the fan, a similar course of the Murti River to the south–southwest, and that of the Neora River to south are indicative of the presence of young lineaments in this region that are oriented transverse to the main Himalayan orogen. Several such lineaments can be picked up in the DEM images (see ‘L’ in Fig. 2). These lineaments

Please cite this article as: Kar, R., et al., Morpho-sedimentary characteristics of the Quaternary Matiali fan and associated river terraces, Jalpaiguri, India: Implications for climatic controls, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.05.014

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Fig. 4. Hypsometric curves of the catchment areas of Neora and Murti Rivers. The diagram separately shows the area: elevation relationship for each of the three different orders of streams in the catchments of the two rivers. Note both the convex-upward and sigmoid shapes of the hypsometric curves for the lower order drainages.

are sub-parallel to the Gish Transverse Fault (GTF) (Mullick et al., 2009; Mukul et al., 2014). This transverse fault system appears to be active in this region and an E–W extension at the rate of 10.9 ± 1.6 mm yr−1 has been measured through a network of GPS stations in this area (Mullick et al., 2009). However, a north–south shortening varying from ~11 to 20 mm/yr from eastern Himalaya has been reported by various authors (Jade et al., 2007; Mullick et al., 2009; Burgess et al., 2012). The GPS

stations used in these ground motion calculations include a network of stations from our study area and are consistent with the thrustsense movement across Matiali and Chalsa scarps (Mullick et al., 2009). The calculated Mountain Front Sinuosity Index (Smf) indicates that the mountain near the fan apex is tectonically inactive (Table 1; cf. Keller and Pinter, 1999). The catchment basin of the Murti and Neora Rivers shows that for 1st order streams, the hypsometric integrals

Please cite this article as: Kar, R., et al., Morpho-sedimentary characteristics of the Quaternary Matiali fan and associated river terraces, Jalpaiguri, India: Implications for climatic controls, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.05.014

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Fig. 5. Quaternary geological map of the study area. Note the steep southward dip associated with the Matiali and Chalsa Scarps and their reversal to a gentle northward dip north of the scarps (see also Fig. 2b). Also, note confinement of the younger two terraces within the river valleys south of the Matiali scarps. The fan surface has been marked as T3a, T3b and T3c as explained in the text. Locations of field photographs are also marked in the map.

(Hi) are moderate to moderately high (0.429 to 0.483). Flat hypsometric curves for higher order streams, and the pronounced convexity of some of the 2nd order streams imply that the smaller order streams are of relatively recent origin. The entire catchment basins, however, tend to be more mature, with Neora catchment being slightly less so (c.f., Sakaguchi, 1969, 1971; Perez-Pena et al., 2009). The Smf value, on the other hand, implies that there has been no major tectonic activity reshaping the mountain front. The southward sloping steep faces with gently northward dipping surfaces immediately north of the Chalsa and Matiali scarps and the south-dipping stratification of the underlying sediment below the scarps together define the asymmetric anticlinal folds (Chakraborty et al., 2005; Guha et al., 2007; Goswami et al., 2012). The surface slope and internal stratification pattern of these folds resembles asymmetric fault propagation folds (Suppe, 1983; Mitra, 1990). In the absence of evidence of surface-breaking faults, disruption of strata or juxtaposition of dissimilar strata, and in the overall context of the proximal foreland setting, we infer, these asymmetric folds as fault-propagation folds where hanging wall sediments were transported southward over north-dipping blind thrusts (Suppe, 1983; Mitra, 1990; Chakraborty et al., 2005; Chakrabarti Goswami et al., 2013) (Figs. 2, 6, 9). Although no surface-breaking thrust has been recorded in the study area,

north-dipping thrust faults and related folds have been reported from the Quaternary sediments, in a trench ~ 5 km east of Chalsa (Kumar et al., 2010). This provides corroborative evidence for our interpretation of thrust-sense movement along the Chalsa scarp. The rising antiforms deflected or terminated smaller gullies whereas comparatively larger channels could downcut through the rising fold axis (cf., Gupta, 1997; Burbank et al., 1999; Van der Beek et al., 2002). The coalescence of the south flowing first order channels and sharp bends of the 2nd order channels close to the Matiali scarp clearly indicate antecedence of these channels to the formation of the Matiali scarp. Had the 1st order channels been younger than the Matiali scarp both north flowing and south flowing streams would have been common on the T3b surface showing the influence of a preexisting topographic high. Near the Chalsa scarp, in addition to fan sediments, the T2 terrace sediments were also affected by the folding whereas the T1 sediments are unaffected by folding indicating that T1 formed after the development of the Chalsa fold. The confinement of the paired terraces (T1 and T2) within the Murti, Kurti and Neora River Valleys south of the Matiali scarp (Fig. 5) shows that their accretion post-dates both the emplacement of the fan and the development of the Matiali folding. The development of terraces within the river valleys represents younger events of aggradation when

Please cite this article as: Kar, R., et al., Morpho-sedimentary characteristics of the Quaternary Matiali fan and associated river terraces, Jalpaiguri, India: Implications for climatic controls, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.05.014

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Fig. 6. Photograph showing the asymmetrically folded fan surface at the Matiali scarp. The blown up images show the underlying southward dipping, gravelly fan sediments.

the sediment to water ratio changed favourably, forcing the fluvial systems to fill up the valleys. 4. Sedimentology 4.1. Physical characteristics of fan sediments The alluvial fan in the Matiali–Chalsa area comprises unconsolidated Quaternary sediments that are exposed along incised riverbank sections. Generally, the sediments are clast- to matrix-supported gravels with subordinate pebbly sand, silt and clay. In the proximal part, the sediment comprises megagravels with blocks larger than 7 m

(terminology of Blair and McPherson, 1999). The distal end of the fan, and the T1 and T2 terraces comprise finer pebble gravel, pebbly sand and volumetrically minor silt–clay. T3 sediments are exposed in the upper-middle part of the Matiali fan. In these exposures, sub-angular to sub-rounded clast- to matrixsupported, cobble-, boulder- and block-bearing megagravel with a coarse pebbly sandy matrix comprises the fan sediments (Fig. 10). The colour of the matrix varies from dark yellowish orange (10YR6/6) to moderate brown (5YR4/4). Exotic, several-metre long boulders float in a pebbly sand matrix. These coarse boulder beds alternate with pebbly very coarse sand layers, defining metre-scale crude bedding in the succession. A 70 m thick exposure is logged and displays a coarsening-up

Fig. 7. Southward dipping sandy gravel deposit near Chalsa scarp, west bank of the Kurti River. Asymmetrically folded terraces T3c and T2 are seen in the background. Note an erosionally based, small patch of sub-horizontal coarser gravel on top of T2, marked T2a.

Please cite this article as: Kar, R., et al., Morpho-sedimentary characteristics of the Quaternary Matiali fan and associated river terraces, Jalpaiguri, India: Implications for climatic controls, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.05.014

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beds, probably deposited by supercritical sheet-floods on a steeply sloping alluvial fan surface (Blair and McPherson, 1994). In the distal part of the Matiali fan (T3b) imbricated pebble–cobble gravel and parallel stratified sheet-flood sandy gravels (Figs. 11, 12) imply a decrease in the slope and consequent change in flow character and competence. Frequent occurrences of parallel stratified, sheetflood deposits in the distal regions imply an environment similar to sandskirt facies of an alluvial fan margin (Blair and McPherson, 1994). Sheet-like sandy gravels of T2 and T1 with well-developed clast imbrication, suggest a streamflow origin for these sediments. A locally developed coarsening upward grain size trend suggests development of a telescopic fan like geomorphic form (Colombo, 2005; Meetei et al., 2007). A continuous change of facies from proximal to distal parts of the Matiali fan reconfirms that the T3a, T3b, and T3c belong to a progressively distal part of the same fan surface and therefore, their demarcation as separate geomorphic surfaces (Samsing, Matiali, Rangamati, etc. of Nakata, 1989) is unwarranted. This continuous facies transition and the absence of syn-depositional deformation (such as intraformational unconformities and sharp changes in facies) further imply that a scarp-forming thrust system became active after the deposition and abandonment of the fan or the deposition of T2 sediments. Poorer consolidation of terrace sediments, reduced clast size, stream flow origin of these deposits and confinement of the terraces within the incised river valleys (Fig. 6) are consistent with the later origin of these terraces through stream flow processes after the fan was abandoned and rivers incised valleys in it. 5. Optical dating of fan sediments

Fig. 8. Drainage map of the Matiali fan. The 1st order channels show a radiating pattern north of the Matiali scarp. Note the merger of smaller streams with the larger ones and the sharp bend of these streams as they cross the scarps indicating that the formation of the scarps postdates the channels. Note the straight course of the 2nd and 3rd order channels in the eastern part of the fan controlled by the lineament shown in Fig. 2.

trend (Fig. 10). Southwards, the terrace T3 (T3b surface, Fig. 5) shows a decrease in clast size and fining upwards cobble–boulder gravel beds with well developed imbrication (Fig. 11). Some of the distal fan successions are dominated by horizontally stratified pebbly sand or pebble gravel (Fig. 12). The thickness of the T1 terrace is about 5 to 10 m and maximum elevation of T2 is about 20 m from the riverbed. Sediments of these terraces (Fig. 7) are grey to brownish grey and are less consolidated compared to T3 sediments. The sediment comprises gravel with interlayered beds of coarse to very coarse pebbly sand. Clast size varies from pebbles to small boulders and is finer than that comprising the Matiali fan. In most places, the fabric in these deposits is clastsupported and imbricated gravels that reveal a flow direction towards south. In the proximal part of the T2 deposits, a disorganised fabric is also observed locally. Sandy beds are more common in these river terraces and both coarsening and fining upward grain size trends are observed. 4.2. Sedimentary environments and tectonic implications Thick, disorganised megagravels with outsized clasts (Fig. 10) in the proximal fan areas were emplaced as mass-flow deposits (Nemec and Postma, 1993; Blikra and Nemec, 1998; Sohn et al., 1999). Sediment gravity flow deposits alternate with units of stratified pebbly gravel

Samples for optical dating of fan sediments from the study were collected in specially designed aluminium pipes (Chandel et al., 2006). The samples were treated with 10% HCl and 30% H2O2 to remove carbonate and organic matter respectively, and then dry sieved to obtain grain sizes of 150–210 μm for samples PCMA 2, 6, and 7, and ELB-1 and 90–150 μm for PCMA 3. The underlying principle and methodology of the dating technique are given in the supplementary section (Appendix A; see also Singhvi et al., 2001). Samples PCMA 2 and 3 were collected from near the base and top of T1 and T2 respectively as exposed in the downstream part of the Neora River section, whereas PCMA 6 was collected from the base of T1 sediments in the Kurti River section (Fig. 2). Samples PCMA 7 and ELB 1 were collected from the low-gradient Quaternary sediments from the Chaiti and Churanthi River sections, occurring outside the Matiali fan (Fig. 2). The results obtained from these samples are shown in Table 2. Sediments of the T2 surface were deformed close to the Chalsa scarp whereas T1 sediments remained undeformed, and therefore, the OSL dates from the top of T2 and the base of T1 constrain the age of deformation along the Chalsa blind thrust between 11 and 6 ka (Table 2). The OSL dates of the PCMA-7 and ELB-1 samples indicate that the sedimentation south of the Matiali fan ceased at ~ 5 ka. These sediment bodies thus accreted almost contemporaneously with the river terraces and are much younger than the Matiali alluvial fan sediments. 6. Discussion A series of Quaternary high-gradient alluvial fans occur all along the Himalayan mountain-front interspersed with low-gradient large megafans and fluvial deposits (Singh, 2004; Tandon et al., 2008; Chakraborty, 2010). A number of workers have worked on these fans and the associated Quaternary terraces in the western Himalayan sector, trying to decipher the interplay of tectonics and climate in the aggradation-incision of the fan–terrace system (Singh et al., 2001; Malik and Nakata, 2003; Suresh et al., 2007; Sinha et al., 2010; Singh and Tandon, 2010). Similar analysis in the piedmont deposit of eastern Himalaya is less common. Occurrence of two large megafans (Tista and Kosi megafans) in the eastern Himalaya becomes all the more important

Please cite this article as: Kar, R., et al., Morpho-sedimentary characteristics of the Quaternary Matiali fan and associated river terraces, Jalpaiguri, India: Implications for climatic controls, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.05.014

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Fig. 9. (a) A tracing from the photograph showing disposition of terraces north of the Chalsa scarp along the west bank of the Kurti River. Note deformed T3c terraces, southward dipping strata within T2 terrace and undeformed sub horizontal bedding in T1. (b) A transverse profile, prepared from DEM data, at the location of 9a showing disposition of terraces on two banks of the Kurti River.

as their origin has been debated and a number of different hypotheses invoking both climatic and tectonic forcing have been proposed to explain the origin of these megafans (Burbank, 1992; Gupta, 1997). Sinha et al. (2005) have shown that stream power and sediment supply of the eastern and western Himalayan foreland basin differ significantly, resulting in contrasting behaviour of the streams that probably results in the contrasting geomorphology of these two areas. They showed that while in the western Himalaya, the streams are being incised, such incision is absent in eastern Himalaya and large megafans are common in the eastern part. Sinha et al. (2005) suggested higher rainfall and higher rate of crustal convergence resulting in higher rate of uplift in the eastern Himalaya as the main reason for this change in geomorphic processes. It is against this backdrop that the study of the alluvial fan–terrace system in the eastern Himalaya assumes importance. The coarse grained fan, associated river terraces and the reported neotectonic features (Nakata, 1989; Kumar et al., 2010; Chakrabarti Goswami et al., 2013) in the Matiali area in the eastern Himalaya provide the opportunity to examine the relative role of on-going tectonics and climate change in shaping the mountain-front landscape. Based on the geomorphic evidence from the fan–terrace system, sedimentological study of the Quaternary deposits and OSL dates, this study suggests, for the first time, that the climate was the main driver in shaping the mountain-front geomorphology in this part of the eastern Himalaya, rather than hinterland tectonics. The geomorphic features indicate that the Matiali fan is now abandoned and the streams of different orders in this region are presently incising creating valleys within or at the margin of the fan. The average slope of the fan surface is about 2° to the south, but the surface is traversed by three steeply (9°–17°) sloping E–W scarp faces around Chalsa, Matiali and Samsing. Immediately north of the Chalsa and Matiali scarps, fan surfaces have a low (0.68° to 1.56°) northerly slope. A steeper southern slope and a less steep northerly slope of the fan surface around the Chalsa and Matiali scarps and congruent southward tilted strata underlying the scarps together define the asymmetric antiformal folds developed in the fan and older terrace deposits. Since no evidence of disruption, displacement or superposition of dissimilar strata has been seen in the study area, we instead of describing

these steeply dipping scarps as faults (cf. Nakata, 1989), infer these scarps as asymmetric limbs of fault-propagation folds developed on the hanging wall of north-dipping blind thrusts (cf., Suppe, 1983; Mitra, 1990; Chakraborty et al., 2005; Figs. 5, 6, 9). Kumar et al. (2010) have documented a low-dipping thrust and related fold in the Quaternary sediments in a trench excavated about 5 km east of the study area situated on the lateral extension of the Chalsa lineaments. Their documentation corroborates our interpretation of Matiali and Chalsa folds as related to movement along north-dipping thrusts. However, in our study area, the thrust is not ground breaking and the geometry of the fan surface and underlying tilted strata appears to be best explained as fault-propagation folds related to the movement along blind thrusts. Chakrabarti Goswami et al. (2013) also inferred these scarps to represent “ramp anticlines” developed over blind thrusts. A similar scarp face at Samsing as also marked by Nakata (1989) as a fault, probably developed in the same way. However, the lack of suitable exposures prevents us from collecting detailed information. Continuous facies change from the proximal part of the fan to its distal end, as documented in this study, shows that the succession belongs to a single alluvial fan. Thus, the recognition of different parts of the same fan surface separated by three scarps (T3a–T3c; Samsing, Matiali, Rangamati, etc.) as separate geomorphic surfaces by earlier workers is misleading. Sedimentological evidence of uninterrupted sedimentation of the entire fan also indicates that all the fault-propagation folds in the study area postdate the deposition of the fan and did not directly affect its sedimentation. There is no evidence of any local unconformity/disconformity separating packages of strata, abrupt facies change or termination of strata against a tectonic feature, that is in congruence with our interpretation that the thrust movement along the Matiali and Chalsa scarps post-dates the fan sedimentation. The larger streams of this area incised through the rising antiform, responding to the deformation through abrupt bending of their courses near the scarps. A subsequent event of high sediment supply changed these incising streams to aggradational mode forming a couple of aggradational terraces. A distinctly different colour, poorer consolidation, finer grain size, streamflow origin, and particularly, the spatial confinement

Please cite this article as: Kar, R., et al., Morpho-sedimentary characteristics of the Quaternary Matiali fan and associated river terraces, Jalpaiguri, India: Implications for climatic controls, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.05.014

R. Kar et al. / Geomorphology xxx (2014) xxx–xxx

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Fig. 10. A log showing a coarsening upward succession of the T3a gravelly sediments. Photograph shows the details of a part of the succession. Note in the photo megagravel with large blocks is separated by finer gravel with horizontal strata.

within incised rivers valleys south of the Matiali scarp indicate that the T2 terrace postdates the abandonment of the fan and the growth of the Matiali antiform. Deformation of T2 deposits suggests that fault-propagation fold near the Chalsa scarp postdates the deposition of T2. The undeformed T1 terrace formed after this deformational event and imply a second phase of aggradation in the rivers.

The following succession of events (Fig. 13 a, b) can be inferred from the geomorphology of the Matiali fan and associated terraces: 1) Deposition of the Matiali fan (aggradation event 1 or A1). 2) Abandonment and incision of the Matiali fan (Incision event 1 or I1). Present day orientation of the Neora, Murti and Kurti Rivers possibly

Fig. 11. Imbricated boulder gravel in the medial part of the Matiali fan (T3b).

Please cite this article as: Kar, R., et al., Morpho-sedimentary characteristics of the Quaternary Matiali fan and associated river terraces, Jalpaiguri, India: Implications for climatic controls, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.05.014

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R. Kar et al. / Geomorphology xxx (2014) xxx–xxx

Fig. 12. Photograph illustrating the parallel stratified pebbly sand of sheet flood origin in the lower reaches of the Matiali fan (T3c).

3)

4) 5)

6) 7)

took place during this period and smaller gullies started forming on abandoned fan surface. Movements along blind thrusts formed Samsing and Matiali scarps and related fault-propagation folds (deformation events 1 and 2 or D1 and D2). Sediment aggradation within the incised river valleys south of Matiali scarp formed the T2 terrace (A2). Movement along another blind thrust further south produced the Chalsa scarp deforming the sediments of T3 and T2 in the vicinity into another asymmetric antiform (D3). T2 terrace was incised (I2) and minor terrace T2a (A3) and subsequently T1 accreted after incising T2a (I3 and A4). T1 was incised and the rivers occupied their present day position (I4).

The oldest 14C date reported from the basal part of the fan succession is around 33,875 ± 550 Ybp (Guha et al., 2007). Three OSL ages from the

top of T2 and base of T1 yield dates ranging from 11 ± 2 to 6.1 ± 0.9 ka (PCMA-2, -3, -6, Table 2). OSL dates from two adjacent low gradient sediment lobes accreting south of the Chalsa lineament is 4.7 ± 0.5 and 4.8 ± 0.6 ka (samples PCMA 7 and ELB 1 respectively, Table 2, see Fig. 2 for the sample locations). Returning to the tectonics vs climate change debate many of the recent studies have stressed the importance of climate (Harvey, 2005; Bookhagen et al., 2006; Jones et al., 2014). Even at a larger scale, some of the recent studies (Posamentier and Allen, 1993; Blum and Tornqvist, 2000; Goodbred, 2003; Rahaman et al., 2009; Lu et al., 2010; Leeder, 2011) reveal the important role of Quaternary climate change in sedimentary processes and sequence development across continents. According to many workers, climatic forcing is consistent with high sediment fluxes and rapid rates of incision during the Quaternary (Curray and Moore, 1971; Burbank et al., 1996; Einsele et al., 1996; Leland et al., 1998). Different studies of sediment dispersal processes in the Ganges river system at different time and space

Table 2 Optically stimulated luminescence dating of five samples of the study area (Fig. 2). S. No.

Sample

1 PCMA-2 88.77706° E 26.89604° N 2 PCMA-3 88.77696° E 26.89557° N

3 PCMA-6 88.79738° E 26.88735° N

Depth (m) Top of T2 terrace

2.5 m below the top of T2

Base of T1 terrace

1.5 m

4

U (ppm)

Th (ppm)

K (%)

CR (µGy/a)

Water (%)

Model

2.3 ± 0.9

11 ± 2

2.1 ± 0.1

191 ± 19

15 ± 5

Mean Min + 2σ

3±1

2.5 ± 0.7

2.0 ± 0.1

19 ± 5

8±2

14 ± 3

1.9 ± 0.1

2.0 ± 0.1

2.4 ± 0.1

214 ± 21

220 ± 22

220 ± 22

15 ± 5

15 ± 5

15 ± 5

PCMA-7 88.68434° E 26.86143° N 1m

5 ELB-1 88.53333° E 26.85° N

4.3 ± 0.5

7±2

1.5 ± 0.2

470 ± 47

15 ± 5

No. of discs

Dose rate (Gy/ka)

Age (ka)

Protocol

Grain size

62 ± 6

21

3.0 ± 0.3

21 ± 3

SAR

180 ± 30

21 ± 3

2

7±1

SAR

180 ± 30

De (Gy)

Wt mean

55 ± 4

21

MEAN

87 ± 2

13

18 ± 2

SAR

180 ± 30

25 ± 3

SAR

120 ± 30

Min + 2σ

39 ± 4

3

11 ± 2

SAR

120 ± 30

Wt mean

88 ± 8

13

25 ± 4

SAR

120 ± 30

Mean

50 ± 5

15

18 ± 2

SAR

180 ± 30

Min + 2σ

17 ± 2

1

6.1 ± 0.9

SAR

180 ± 30

Wt mean

51 ± 4

15

18 ± 2

SAR

180 ± 30

Mean

53 ± 5

20

16 ± 2

SAR

180 ± 30

Min + 2σ

16 ± 1

1

4.7 ± 0.5

SAR

180 ± 30

Wt mean

41 ± 3

20

12 ± 1

SAR

180 ± 30

10.2 ± 0.3

26

3.4 ± 0.4

SAR

180 ± 30

Min + 2σ

2.4 ± 0.1

3

0.8 ± 0.8

SAR

180 ± 30

Wt mean

14.0 ± 0.9

26

4.8 ± 0.6

SAR

180 ± 30

Mean

3.5 ± 0.4

2.8 ± 0.3

3.4 ± 0.3

2.9 ± 0.2

Please cite this article as: Kar, R., et al., Morpho-sedimentary characteristics of the Quaternary Matiali fan and associated river terraces, Jalpaiguri, India: Implications for climatic controls, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.05.014

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Fig. 13. (a) Schematic diagram summarising the history of development of the terraces in the incised rivers on the Matiali fan. A1, A2… = aggradation events 1, 2…, I1, I2… = incision events 1, 2… (b) A cartoon showing disposition of the terraces in the Kurti River close to the Chalsa lineament (see Fig. 9a for field view and Fig. 1 for the location of the Chalsa scarp). Inclined bedding surfaces in T2 denote deformed and tilted sediments of this terrace.

windows have revealed the role of climatic variation during the Quaternary (Goodbred, 2003; Srivastava et al., 2003; Gibling et al., 2005; Suresh et al., 2007; Tandon et al., 2008; Sinha et al., 2010). The last interstadial MIS-3, between 55 and 25 ka (Fig. 14) was characterised by a moderate climate. During this period Himalayan glaciers were extended ~ 10 km beyond their present positions and locally more than 40 km from where they are now (Owen et al., 2002). As the rainfall increased at the end of the MIS-3 prodigious volumes of sediment produced during glacial activity in the upper reaches was transported down, possibly leading to massive aggradation in the lower reaches (Meetei et al., 2007) of the Ganges River system. Even in the tectonically-active study area the climatic signal appears to override the tectonic signal. The period MIS-3 broadly coincides with the aggradation of the Matiali fan deposits, which has been already dated to be younger than 34 ka (Guha et al., 2007). This

date coincides with the periods of high rainfall in MIS-3 (Fig. 14). During LGM (24–18 ka) the climate was cold, arid and there was a significant decrease in river discharge (Van Campo, 1986; Cullen, 1981). This is probably the period when the Matiali fan was abandoned and incised by the Neora and Murti Rivers. Meetei et al. (2007) have correlated the major incisional phase of the upstream Tista valley system with this glacial period. This cold regime was conducive to the production of a high amount of glacial sediments that were not transported downstream at that time. After the LGM the increased insolation strengthened the summer monsoon. During the increased monsoonal precipitation, between 15 and 5 ka, the regional precipitation was very high (Bookhagen et al., 2005; Sanyal and Sinha, 2010; Anoop et al., 2012 and primary references therein), and this caused transportation of significant volumes of sediment from the production zone to the foreland basin and river valleys.

Please cite this article as: Kar, R., et al., Morpho-sedimentary characteristics of the Quaternary Matiali fan and associated river terraces, Jalpaiguri, India: Implications for climatic controls, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.05.014

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the formation of the Matiali scarp predates the formation of the Chalsa scarp: i) The density of radiating 1st order channels is higher north of Matiali scarp than south of it. This would indicate a younger age of the surface T3c that became stable only after the formation of the Chalsa scarp; ii) The occurrence of T2 south of the Matiali scarp indicates that the terrace is younger than the Matiali scarp. Again T2 has been deformed in the vicinity of the Chalsa scarp, implying that the Chalsa scarp is younger than the Matiali scarp; iii) The existing model for the Himalaya and its foreland basin formation predict progressive southward progradation of the thrust front (Gansser, 1964; Hodges, 2000) and is consistent with the younger age for the Chalsa scarp occurring south of the Matiali scarp.

Fig. 14. Summary of climatic fluctuations in Asia as recorded from different proxies. Major aggradational events in the study area coincide with phases of intensified Asian summer monsoon. Modified after Gibling et al. (2005), Bookhagen et al. (2005), and Anoop et al. (2012).

The dates of T1 and T2 ranging from 11 to 6 ka coincide with this wet period and we infer that climatically induced sediment supply was the reason for the aggradation of terraces in the river valleys during a transitional regime from a cool to a warmer period. A low-gradient sediment lobe south of the Chalsa scarp yielding a date of about ~ 5 ka broadly coincides with the same wet period and was contemporaneous with T1. However, in congruence with the climate model over this period of several thousand years, sediment supply and water budget showed significant fluctuations and were the cause of incision of T2 and T1. Our OSL dates suggest that the deformation that formed the Chalsa scarp and related folds in T3 and T2 could have occurred between 11 ka and 6 ka (age of abandonment of deformed T2 and that of initiation of undeformed T1). Our interpretation of abandonment and incision of the fan during LGM provides a broad age bracket of 24–18 ka for this event. The deformational event that formed the Matiali scarp involving fan sediments thus should be younger than 24 ka. Several lines of evidences listed below indicate that

Assuming, on the basis of this evidence, that the Matiali scarp is older than the Chalsa scarp, the movement along the Matiali thrust probably occurred between 11 and 24 ka. However, from the available data it is difficult to obtain any reasonable age constraint for the formation of the Samsing scarp. Tectonics is the main driving mechanism in creating the Himalayan collisional orogen and a flexure-driven peripheral foreland basin in front of it. Tectonic movement (probably along MFT) created the modern foreland basin (Wesnousky et al., 1999) and created the accommodation space for the deposition of the Matiali fan and other fans along the proximal part of the eastern Himalayan foreland. Two large megafans (Kosi and Tista) having low slopes (≤ 0.05°), comprising medium to fine sand, occur side by side with a large number of smaller high-gradient (av. slope ≈ 2°) alluvial fans. These smaller alluvial fans consist of boulder gravel. Lateral coexistence of such varied landforms, in the same basinal setting of proximal foreland, reflects the geomorphologic controls on the development of these depositional forms, rather than tectonic influence (Fraser and DeCelles, 1992; Chakraborty, 2010). Transition of the streams of Matiali area from an incisional regime to an aggradational regime, as necessary for the formation of aggradational terraces T2 and T1, requires changes in the sediment to water ratio of the fluvial system. Such change cannot be brought about by local uplift related to fault-propagation folds within the Quaternary sediments. This change in most of the cases of Quaternary river terraces all over the globe is strongly correlated with climatic fluctuation (Kar and Chakraborty, 2014). Analysis and careful dating of terraces in many continents have clearly established this (cf., Pratt et al., 2002; Bookhagen et al., 2006; Pan et al., 2007). Similar climatic forcing is also evident from the fan–terrace system of the eastern Himalayan proximal foreland setting. We show here that the aggradational events on the Matiali fan and associated terraces have a good correspondence with the wetter period of MIS-3 and MIS-1 when an increased summer monsoon carried a large sediment flux to the basin overpowering the rivers, causing them to aggrade. Abandonment and incision of the fan and terrace must have been initiated by a decreased sediment to water ratio during periods of weaker monsoon and increased glacial activity. Deformational episodes related to movement along blind thrusts deformed the fan/terrace sediments but they did not control the formation of the river terraces. This study thus establishes from eastern Himalaya, the important role played by the late Quaternary climate oscillations in the development of the proximal mountain front succession and has an important implication for the stratigraphic development of the modern foreland basin. 7. Conclusions • The Matiali fan, one of the coarse-grained mountain-attached fans at the foothills of the Darjeeling Himalaya, was built by a combination of mass flow, sheet flood and stream flow processes.

Please cite this article as: Kar, R., et al., Morpho-sedimentary characteristics of the Quaternary Matiali fan and associated river terraces, Jalpaiguri, India: Implications for climatic controls, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.05.014

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• The Neora, Kurti and Murti Rivers are incising the abandoned fan. In addition, a network of 1st order streams sculpts the abandoned fan surface. A radial pattern of these first order streams reflects the convex upward geometry of the fan surface and is unrelated to the megagravel depositing fan–paleochannel system. • Three east–west trending scarps, namely the Samsing, Matiali and Chalsa scarps cross the fan surface and represent the steeper asymmetric limb of the fault-propagation folds formed over blind thrusts. Several lineaments that transverse to the mountain front have affected the fan body and control the localised course of the stream channels. These transverse lineaments are sub-parallel to the Gish Transverse Fault, across which extensional crustal motion has been documented. • Deflection and coalescence of the first order streams across the Matiali and Chalsa scarps reflect the antecedence of the small channels to the fault-bend folds. • Two paired, aggradational terraces are present south of the Matali scarp, within the incised river valleys of the Neora, Kurti and Murti rivers. The terraces and low-gradient sediment lobes occurring south of the Chalsa scarp represent younger aggradational events of this area. • Based on available dates and the succession of geomorphic and deformational events, we suggest the initiation of the Matial fan around 34 ka, coinciding with the intensified summer monsoon of MIS-3. The fan was probably abandoned and incised during LGM (24–18 ka). Renewed terrace aggradation in the river valleys and alluvial sediments south of the fan, coincide with an increased monsoon strength during MIS-2 and 1 (15–5 ka). Formation of the Chalsa scarp took place between 11 and 6 ka and the Matiali scarp appears to have formed between 24 and 11 ka. • The study reveals that the deposition and incision of the fan– terrace system in this part of the eastern Himalaya was driven by climatic fluctuations; on-going tectonics is responsible for the development of fault-propagation folds involving the Quaternary deposits but did not directly influence the sedimentation of the fan–terrace system.

Acknowledgements The facilities for this research were provided by the Indian Statistical Institute, Kolkata. We are grateful to Suchana Taral, Research Fellow, ISI for her considerable support during the final preparation of the manuscript. AKS acknowledges the Department of Science and Technology (DST), Government of India for the J C Bose fellowship. Comments from two unknown reviewers and untiring efforts of the Guest Editor Rajiv Sinha and Geomorphology Special Issue Editor Adrian Harvey made significant improvements in the manuscript. Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.geomorph.2014.05.014. These data include the Google map of the most important areas described in this article. References Anoop, A., Prasada, S., Basavaiahc, N., Brauera, A., Shahzadd, F., Deenadayalan, K., 2012. Tectonic versus climate influence on landscape evolution: a case study from the upper Spiti valley, NW Himalaya. Geomorphology 145–146, 32–44. Blair, T.C., McPherson, J.G., 1994. Alluvial fans and their natural distinction from rivers based on morphology, hydraulic processes, sedimentary processes, and facies assemblages. J. Sediment. Res. 64, 450–489. Blair, T.C., McPherson, J.G., 1999. Grain-size and textural classification of coarse sedimentary particles. J. Sediment. Res. 69, 6–19. Blikra, L.H., Nemec, W., 1998. Postglacial colluvium in western Norway: depositional processes, facies and palaeoclimatic record. Sedimentology 45, 909–960.

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Please cite this article as: Kar, R., et al., Morpho-sedimentary characteristics of the Quaternary Matiali fan and associated river terraces, Jalpaiguri, India: Implications for climatic controls, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.05.014

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Please cite this article as: Kar, R., et al., Morpho-sedimentary characteristics of the Quaternary Matiali fan and associated river terraces, Jalpaiguri, India: Implications for climatic controls, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.05.014