Luminescence chronology of alluvial fan in North Bengal, India: Implications to tectonics and climate

The collision of the Indian and the Eurasian plates resulted in north-south contraction of the lithosphere and development of southward propagating fold and thrust belt (FTB) (Molnar and Tapponnier, 1975; Molnar, 1984; Guha et al., 2007). Himalayas are one of the few longest mountain ranges in the world, comparable with Andes, Rocky Mountains, Great Dividing Range etc. The Himalayan FTB has been divided by northward dipping major boundary thrusts; Main Central Thrust (MCT) separates the crystalline Higher Himalayas from the metamorphosed Lesser Himalayas, Main Boundary Thrust (MBT) separates the Lesser Himalayas from the sedimentary rocks of the Siwaliks and Main Frontal Thrust (MFT) being the southernmost expression of FTB at mountain front separates the Siwaliks from the Quaternary deposits (Gansser, 1964, 1981). Himalayas provide a natural laboratory to the earth scientist trying to understand the balance between uplift, incision and erosion in an actively deforming orogen, resulting in topographic evolution on the 10⁵–10⁷ years timescales (Lavé and Avouac, 2000; Wobus et al., 2005). In present day scenario, the convergence between the Indian and the Eurasian plate is majorly accommodated by the movement on the MFT (Lavé and Avouac, 2000; Kumar et al., 2001; Malik et al., 2003; Malik and Nakata, 2003; Lavé et al., 2005). The MFT has mappable surface expressions in piedmont zones of Uttranchal, Darjeeling, and Arunachal Pradesh Himalayas (Raiverman et al., 1990; Valdiya et al., 1992; Srivastava and John, 1999; Sarkar and Som, 2001; Mukhopadhyay and Mishra, 2004). It is a commonly accepted hypothesis that either the thrust tip is blind in most of the areas or its trace is buried by rapid accumulation of sediments (Raiverman et al., 1990; Valdiya et al., 1992; Srivastava and John, 1999; Mukul 2000; Sarkar and Som, 2001; Mukhopadhyay and Mishra, 2004). In the Darjeeling foothill Himalayas, to the east of the Tista River, the MFT has discontinuous and nonlinear surface expressions (Nakata, 1989; Guha et al., 2007; Goswami et al., 2012).

Himalayan foreland basins are suitable to understand the role of climate and tectonics in alluvial fan sedimentation, as this region has been affected by both the factors (Nakata, 1972; Ruddiman et al., 1989; Valdiya et al., 1992, 1993; Singh et al., 2001; Goodbred, 2003; Srivastava et al., 2003; Gibling et al., 2005; Sinha et al., 2005). Singh (1996a, 1996b) suggested that the mega fans formed in the initial part of humid phases preceded by a long period of arid climate, which helped in generating sediments. Srivastava et al. (2003) attempted luminescence chronology of Yamuna Ganga mega fans with multiple aliquot additive dose method and reported its formation during 26–22 ka, but did not rule out the possibility of insufficient bleaching and hence an overestimation of the ages. Jaiswal, (2006, unpublished thesis) reassessed the dates of (Srivastava et al., 2003) using Single Aliquot Regeneration (SAR) method and suggested that the mega fan sedimentation was during the humid phase (~14–16 ka) preceded by Last Glacial maxima (LGM), which supports the hypothesis of (Singh 1996a, 1996b). Under wet conditions rivers not only flush out the produced sediments but also incise the bedrock forming various levels of terraces and intense uplifting in tectonically active areas is reflected by large number of rockcut terraces (Starkel, 2003). The study of quaternary deposits may provide important clues to the sedimentation style at long mountain chains spread all over the world which are similar to Himalayas. Thus the study of alluvial fans and fluvial terraces may provide information on the river dynamics on regional scale and effect of climate-tectonic interactions on the evolution of these landscapes.

A lot of work has been done in the western Himalayas to understand the mechanism shaping the morphology of the area along with the geochronology (Singh et al., 2001; Srivastava et al., 2003; Suresh et al., 2007; Kumar et al., 2007), whereas such work is less common in Darjeeling-Sikkim Himalayas (Mukul et al., 2007; Kar et al., 2014). Current study focuses on an alluvial fan in Darjeeling-Sikkim Himalayas, drained by the rivers like Neora, Kurti and Murti in the northern part of West Bengal, India (Fig. 1). A few studies have been done earlier in the region to understand the formation of this fanbut they lack sufficient geochronological data. The alluvial fan east of the Tista River in the Mal - Murti interfluves area is named as the Matiali fan (Fig. 1). At mountain fronts in Himalayas, the Indian monsoon system plays an important role in regulating the sediment supply and its transportation, thus controls the evolution of the fan system. In addition to this the Matiali fan has been affected by several tectonic movements along the Chalsa and Matiali faults. Thus, this fan provides an opportunity to explore climate-tectonics interactions.

Guha et al. (2007), based on their two radiocarbon dates, one near the Gorubathan thrust and another near the Chalsa thrust attempted to explain the evolution of fan. However, the C-14 age of 33,875 ± 550 Ybp near the Gorubathan thrust (Guha et al., 2007) gives only a lower bound and shows that the fan formation started before that. This date have been found much lower than other radiocarbon age, reported to be ~46.4 ka by Ghosh et al., (2015) from a terrace in the distal part of the fan. However these dates were taken from proximal and distal part of the fan therefore cannot be compared. Kar et al., (2014) provided 3 optical ages from the terraces on the Neora and Kurti rivers in the distal part of the fan which are young ages (all below 11 ka). This shows that there is an ambiguity in terms of chronology. Goswami et al., (2013) using total station and remote sensing suggested that this fan is affected majorly by tectonics. Kar et al., (2014) discussed the evolution of the fan and linked it with the past climate neglecting any role of tectonics, in general. As the fan is located in between MBT and MFT, the development of fan is controlled by both climate and tectonics. However, which is a dominant factor, is still a controversy. The detailed chronology of various fans located along the MFT may be helpful in solving these issues. The Matiali fan is a complex geomorphic entity which has been under continuous deformation and needs detailed chronological study to frame these tectonic movements. The present work provides a detailed chronology of the area using optical dating and attempts to remove the ambiguity in geochronological data and thus, helps in understanding the geomorphic evolution of the alluvial fan with time.

The study area is an alluvial fan bound by major rivers, the Mal in the west and the Murti in the east, which originate in the Higher Himalayas. The alluvial fan is dissected by many smaller order tributaries of these major rivers, e.g. the Kurti and the Juranti which originate in the piedmont zone itself (Fig. 1c). Two East-West trending scarps, the Matiali scarp in north and the Chalsa scarp in the south considered as the splay of the MBT and the MFT respectively owe their origin to the movement on blind thrust below these scarps (Nakata 1989; Guha et al., 2007) (Fig. 1). One more scarp named as the Gorubathan scarp exist to the north of Matiali scarp but could not be studied due to lack of suitable exposure, also represents MBT (Fig. 1) (Guha et al., 2007). Nakata (1989) identified three geomorphic surfaces in the Quaternary deposits, viz., Matiali, Rangmati and Samsing in order of their time of formation, Matiali being the oldest and Samsing as the youngest. He also categorized terraces in three groups: (i) Lower River Terrace or Younger River Terrace (LRT or YRT), (ii) Middle river terraces (MRT) and (iii) Higher river terrace (HRT). However, based on pedological studies, Das and Chattopadhyay (1993a, 1993b) considered Matiali and Rangamati to be a single geomorphic surface and Samsing to be the oldest surface. Similar classification was followed by later workers (Guha et al., 2007; Goswami et al., 2012, 2013; Kar et al., 2014). Goswami et al., (2012,2013) classified regional highest surface of the fan as T₄ and subsequent lower surfaces as T₃, T₂ and T₁. The regional slope of the fan is southerly (2.1°), at the head, height is 550 m above mean sea level (MSL) which decreases to 200 m above MSL at toe (Goswami et al., 2012, 2013). Goswami et al., (2013) reported that the Matiali and the Chalsa scarps are manifestation of antiforms over two north dipping blind thrusts. The regional fan surface T₄ has been dissected and incised by many rivers like Neora, Murti, Kurti and Juranti forming lower terraces. T₃ occurs as raised terrace north of the Matiali scarp on both the banks of the Murti, Kurti, Juranti and only on the east bank of the Neora river. South of the Matiali scarp, raised terraces on the banks of Neora, Kurti and Murti are designated as T₂ and T₁. Terrace T₂ has an average height of 200 m above MSL. T₁ is above recent flood plains and has an average height of 180 m above MSL. Matiali scarp is highest (60 m) on the eastern bank of the Neora River and its height decreases eastward and the scarp dies out on the western bank of the Murti River after taking a north-east bend (Fig. 1). Chalsa scarp is 90 m in the Neora-Murti interfluve area, traceable from the Chel River to Mal River and in the Murti-Jaldhaka interfluves area also. The higher surfaces show signature of deformation whereas the lower small terraces are undeformed. In addition to E-W trending scarps, there are transverse faults trending NNE-SSW and nearly N-S along which the present day Murti and Neora rivers flow (Goswami et al., 2012, 2013; Ghosh et al., 2015). The radial drainage pattern, north of the Matiali scarp has been modified by later tectonism (Goswami et al., 2012).

Optically stimulated Luminescence (OSL) dating provides the date of the event of last exposure of sediments to sunlight, thus giving burial ages (Huntley et al., 1985; Aitken, 1998). The method is well suited for aeolian deposits as these samples are supposedly well bleached in daylight before burial, however in case of fluvial deposits due to various factors e.g. sediment depth, attenuation of daylight in water column and turbidity of the water along with transport depth, some part of the sediments may or may not be well bleached, thus giving overestimated ages. With the application of Single Aliquot Regenerative (SAR) protocol of OSL dating (Murray and Wintle, 2000) one can have a better insight into the bleaching of sediments and thus can provide better estimate of ages. Luminescence dating technique using SAR has been widely used in establishing the chronology of fluvial deposits in India and in particular to Himalayas (Srivastava et al., 2003; Suresh et al., 2007; Jaiswal et al., 2008, 2009; Juyal et al., 2010; Ray and Srivastava, 2010).

The samples were collected from six locations; these sites were chosen on the basis of relative stratigraphy of different terraces (Figs. 2, 3). Among these the location of the S-5 is outside of the Matiali fan near the Taljhora scarp, close to Taljhora stream (Fig. 1d) was collected to check with independent age control provided by Guha et al., (2007). Various tests were performed on S-5 for optical dating (Fig. 4). The location of sampling sites follows and is also summarised in Table 1.

OSL ages obtained from the sediments of alluvial fan and river terraces constrain the age of different depositional and incision phases. The results of OSL dates have been summarized in Table 2. The quartz obtained from the sample JRM-1, near Matiali scarp from the T₄ surface, has the oldest depositional age of 171 ± 18 ka. Further west of this location from NRM section, sample NRM-1 was collected from the terrace T₃ on the eastern bank of Neora river. It gave a depositional age of 59.7 ± 16 ka. Sample NRM-2, from the western bank of Neora river (T₄) has a depositional age of 84.6 ± 8.6 ka.

In KRC section on the banks of Kurti river, KRC-1 from the base of T₂ terrace has a depositional age of 47.7 ± 10.7 ka; KRC-2 from the top of this terrace has an age of 41.4 ± 10 ka, bracketing the event of sedimentation of terrace T₂. At the same location, KRC-3 has depositional age of 80.8 ± 4.7 ka, representing the material from the alluvial fan (T₄ surface). KRC-4, located at the southern margin of Chalsa fault, shows horizontal bedding is from the T₁ terrace and has depositional age of 28.6 ± 8.5 ka. These beds are unaffected from the deformational events which modified the T₂ terrace. NRC-1, NRC-2, and NRC-3 were collected from the NRC section. NRC-1 from the T₄ surface has depositional age of 89.1 ± 17.3 ka, NRC-2 lies stratigraphically above NRC-1 and gives an age of 72.6 ± 19.9 ka. NRC-3 from the T₂ terrace on the western bank of the river is 59.4 ± 18.1 ka old. JRC-1 collected from the JRC section near the confluence of Neora and Juranti Rivers, is the material from the T₄ and has OSL age of 99.03 ± 25.3 ka. JRC-2 from the youngest terrace has deposition age of 5.9 ± 1.3 ka. The OSL age of quartz obtained from the S-5 sample from the adjacent fan located further east of the Matiali and Chalsa scarps near Taljhora is 27.8 ± 7 ka and is in close match with C-14 age of 27,210 ± 240 Ybp provided by Guha et al., (2007).

Guha et al., (2007) suggested that the oldest deposits in the region date 33,875 ± 550 Ybp. Goswami et al., (2013); Kar et al., (2014) took this date as a bench mark and restricted the formation of the alluvial fan to this time period. However the oldest sample dated by OSL in our study gives an age of 171 ± 18 ka, suggesting that the formation of this alluvial fan started much before than 33,875 ± 550 years.

The JRM-1 from the section along the bank of the Juranti river gives a depositional age of 171 ka suggesting that the formation of the fan started before 171 ka. This formation was accompanied with a strong fluvial activity as boulders of several meters (2–3 m on an average) diameter and a few up to 7–8 m were found in the proximal part of the fan suggesting strong monsoon. The sediments in the proximal part of the fan are disorganized; lack any stratification, suggesting mass-flow deposits. The Neora river section just below the Matiali scarp from the western bank is polymictic conglomerate with angular pebbles having age of 84.6 ± 5 ka (NRM-2). There is a large gap of ages after 171 ka. This could be due to the tectonic movement on the Matiali thrust the sediments near the Juranti section (JRM-1 dated 171 Ka) have been pushed up and are exposed for sampling, while the samples collected from the Neora river section (NRM-2 dated 84.6 ka) near Matiali (to the south of the scarp) are not affected much by this tectonic movement as shown by horizontal beds. However, sediments between these dates may be present but not accessible. Also, there is a possibility that alluvial fan deposits are not continuous deposit at one place but spread over the whole fan area, which could be one of the reasons for gap.

The samples collected from the T₄ in the distal part of the fan near the Chala scarp give ages of 99 ka (JRC-1), 89 ka (NRC-1), 81 ka (KRC-3) and 72 ka (NRC-2) showing that the deposition of the fan continued during this time and the Matiali surface and the Rangmati surface (to the south of Matiali) are the facies variation of the alluvial fan and are not different surfaces as defined by Nakata (1989). The deposition on the fan continued till 72 ka or a little beyond as suggested by the date of the sample NRC-2. The parallel stratified beds of cobble, pebble and sand are seen in the distal part of the fan suggesting sheet-flood deposits. The monsoon weakened around 72 ka limiting the supply of sediments from the Himalayas to the foreland basin, also supported by the work of Rahman et al., (2009). Thus the sediment supply to the basin stopped. Since Kurti and Juranti Rivers originated in the alluvial fan, it is possible that they are synchronous with the fan formation and started incision after the fan formation and have exposed the fan sediments to its present level. OSL ages, thus suggest that alluvial fan formation started before 171 ka and continued till 72 ka at least. After that due to reduced sediment/water ratio, the river system incised the fan. After the deglaciation phase during 63 ka (Ray and Srivastava, 2010), the river got enhanced sediment supply due to increased sediment/water ratio. This helped to make fill terraces in the valley (Pazzaglia, 2013).

The undeformed thick sand beds with coarse sand below the T₃ terrace near the Matiali scarp is dated 59.7 ± 16 ka (NRM-1). Benn and Owen, (1998) reported enhanced summer monsoon during the last interstadial (58–24 ka) in the Himalaya. The formation of this terrace could be attributed to the initial phase of enhanced monsoon from a transition of arid phase to humid phase. This terrace shows no signature of deformation to the north of Matiali suggesting that the formation of this terrace post dated the reactivation on the Matiali scarp. In the north of Matiali, it has an average height of 35 m above the river bed; to the south of Matiali it has an average height of 40 m above the Neora river bed (Goswami et al., 2013). Goswami et al., (2012, 2013) designated the term T₃ to the terrace to the north of Matiali and T₂ to the terrace to the south of Matiali, similar classification of HRT and MRT by Nakata, (1989) and Guha et al., (2007) is not well explained. In our study we took two samples, one from the T₃ terrace to the north of Matiali which gave an age of 59.7 ± 16 ka (NRM-1) and another sample to the south of Matiali near Chalsa from T₂ which gave an age of 59.4 ± 18.1 (NRC-3) ka. Thus we deduce that the time of formation of this terrace is around 59 ka. The sediment/water ratio was high during the beginning of monsoon as there was lot of weathered product (produced during glaciation period) available for transportation so there was an aggradational phase which led to the formation of T₃ and T₂ terraces along the banks of the Neora River.

The Kurti and Juranti Rivers originate within the piedmont zone near the Matiali scarp; these are small rivers and did not respond in the similar manner as the Neora and Murti rivers to climate or tectonic changes, which have their origin in the Higher Himalayas. The Kurti and Juranti rivers probably developed as a product of gully erosion and later modified in a channel as the tectonic activity and monsoonal discharge increased along the Matiali fault. From satellite image, it is clear that Juranti river has changed its course and turned west to meet the Neora River due to the activity along the Chalsa fault (Fig. 1c). A sample at height of 3 m from the Kurti river bed near Chalsa gave an age of 47.7 ± 10.7 ka (KRC-1); these beds are deformed and dip towards south. The dip is not constant, as we move north beds tend to become horizontal, pointing that this tilting is because of tectonic activity on the Chalsa fault. In the same section at a height of about 18 m sediments are dated to be 41.4 ± 10 ka (KRC-2). These beds are not deformed and lie unconformably over the lower deformed beds. Thus we can infer that the movement on the Chalsa scarp started somewhere around 48 ka and stopped by 41 ka. This movement helped in aggradation of the system and formation of terrace. Gully erosion along the T₂ terrace exposed the T₄ surface in contact with T₂ terrace. A sample from this surface gave an age of 80.8 ± 4.7 ka (KRC-3), this is indicative of a fill terrace, i.e. T₂ formed adjacent to T₄.

The sample dated from T₁ terrace along the Kurti River gives an age of 28.6 ± 8.5 ka (KRC-4), this terrace shows no signature of deformation suggesting that these terraces are manifestation of climatic changes. Youngest terrace along the Neora river near its confluence with Juranti river is named as T₁a gives an age of 5.9 ± 1.3 ka (JRC-2) showing that the last phase of aggradation experienced by the fan was around 6 ka and after that the rivers have been continuously incising their valleys.

The following conclusion could be drawn from the study:

The formation of the alluvial fan started before 171 ka and continued till 72 ka covering a time span of nearly 100 ky.