optically stimulated luminescence chronology of terrace sediments of siang river, higher ne...

JOUR.GEOL.SOC.INDIA, VOL.79, MARCH 2012

252 PRADEEP SRIVASTAVA AND D. K. MISRA

Optically Stimulated Luminescence Chronology of Terrace Sedimentsof Siang River, Higher NE Himalaya: Comparison of

Quartz and Feldspar Chronometers

PRADEEP SRIVASTAVA and D. K. MISRA

Wadia Institute of Himalayan Geology, Dehradun - 248 001Email: [email protected]

Abstract: Four levels of terraces located along Siang River, north of Main Central Thrust at Tuting, NE Himalaya aredated using Optically Stimulated Luminescence (OSL). The dating technique is applied using (1) Blue LED stimulationon Quartz (2) Infrared Stimulated Luminescence (IRSL) stimulation on Feldspar at 50 °C and (3) Infrared StimulatedLuminescence stimulation on Feldspar at an elevated temperature of 225 °C. The results indicated that the later twoprotocols on feldspars yielded overestimated ages that suggested incomplete bleaching of luminescence signals in feldspar.The ages derived using quartz suggested a nearly continued valley aggradation from >21-8 ka with three phases ofbedrock incision. The phase of aggradation coincides with a climatic transition from cold and dry Last Glacial phase towarm and wet Holocene Optimum. The bedrock incision phases centered at <21 ka, ~11 ka and ~8 ka indicate towardsmajor episodes of tectonic uplift in the region around Tuting.

Keywords: Optically Stimulated Luminescence dating, Siang River, Terraces, NE Himalaya.

INTRODUCTION

Himalaya, a geologically dynamic and high relief terrainis drained by turbulent river systems like Ganga andBrahmaputra. These rivers are flanked by ubiquitous fluvialterraces that at times bear the record of past sedimentgeneration, fluvial dynamics vis-à-vis climatic changes andtectonic perturbations. Optically Stimulated Luminescence(OSL) dating technique has been widely used by researchersto decipher the chronological framework of these archives.The feldspar and quartz, two of the widely usedchronometers, are generally used in establishing theluminescence chronology of any such kind of morpho-stratigraphic sequence. The fact that under a given spectrumof light optical signal in quartz, as compared to feldspar,resets more efficiently allowed the usage of the former moreacceptably. However, over the past few years, feldspar InfraRed Stimulated Luminescence (IRSL) dating has receivedincreasing attention, principally because of the prospectsof the feldspar luminescence growth to date older sediments(Auclair et al. 2007; Buylaert et al. 2007, 2009; Wallinga et al.2007). The routine IRSL dating protocol that uses a low(usually 30-60 ºC) stimulation temperature, in most caseshas been observed effected from anomalous fading of theluminescence signal at room temperature (Aitken, 1998). Thisrequired the development of several fading correction models

(Huntley and Lamothe, 2001; Lamothe et al. 2003) to providereliable ages. However, all correction models are eitheruntestable or carry inherent assumptions that underminethe variations in fading rates over the geological times(Morthekai et al. 2008). Recently, attempts are made to isolatethe optical signals of feldspar that are less prone to fadingwhere, post IR and post IR elevated temperature signals aretested (Thomsen et al. 2008; Buylaert et al. 2009). Authorshave shown that luminescence intensity of feldspar atelevated temperatures IRSL is significantly higher than thatarising from routinely followed low temperature protocolsand also show relatively low anomalous fading rates.However there are limited number of studies that comparedthe chronologies of quartz and routine and elevatedtemperature feldspar using Single Aliquot Regeneration(SAR) protocols (Buylaert et al. 2009).

In the work detailed in this paper we aim to establishOSL chronology of samples collected from a flight of fluvialterraces located at Tuting, along Siang River (called asBrahmaputra River in the plains), NE Himalaya. We providea comparative account of luminescence ages derived usingQuartz, routine IRSL and post IR-IRSL at an elevatedtemperature on Feldspars. Finally we attempt to provide ameaningful chronology to the terraces and relate the phasesof terrace formation with past climatic changes.

JOURNAL GEOLOGICAL SOCIETY OF INDIAVol.79, March 2012, pp.252-258

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LUMINESCENCE CHRONOLOGY OF TERRACE SEDIMENTS OF SIANG RIVER, NE HIMALAYA 253

GEOLOGICAL BACKGROUND ANDGEOMORPHIC SETTING

The terrace sequence is located in the upper catchmentof Siang River in NE Himalaya at Tuting. The River cutsthrough the Proterozoic to Cenozoic rocks, which occur inthe form of eight distinct thrust bound litho-tectonic units.From S to N, in ascending structural order they are: 1. TheSiwalik Group, 2. The Gondwana Group, 3. The YinkiongGroup, 4. The Miri Group, 5. The Bomdila Group, 6. The Sela

Group, 7. The Tidding Formation and 8. The Lohit PlutonicComplex (Misra and Srivastava, 2009). The southernmostUpper Tertiary fluvial sediments of the Siwalik Group of theSub-Himalaya are thrust over the Brahmaputra Alluviumalong the Himalayan Frontal Thrust (HFT) and aretectonically underlain to the north by the rocks of the LesserHimalaya (Gondwana, Yinkiong, Miri and Bomdila groups)along the Main Boundary Thrust (MBT) (Figs. 1a and 1b).The low grade metasediments of the Miri Group (Jain et al.1974) and low to medium grade metamorphics of the Bomdila

Fig.1. (a) Geological map of the Siang river valley (simplified from Misra and Srivastava, 2009). Note the location of the study area.(b) Geological cross section along A-B suggesting the tectonic position of study area.



Group (Das et al. 1975) show tectonic contact between eachother and tectonically override the Gondwana Group ofsediments. The sedimentary rocks of the Yinkiong Group ofEocene age (Singh, 1993) is exposed in the central part of asyntaxial structure in the form of an antiformal tectonicwindow beneath the Miri Group of rocks, which wrapsaround it. High-grade crystalline rocks (Sela Group) of theHigher Himalaya (Verma and Tandon, 1976) tectonically lieover the low to medium grade metamorphics of the BomdilaGroup along the Main Central Thrust (MCT). Further in thenorth, the serpentine bearing Tidding Formation of theTrans-Himalaya (Thakur and Jain, 1975) overthrusts theSela Crystallines along the Tidding Thrust (TT) and isoverthrust by the rocks of the Lohit Plutonic Complex (LPC)along the Lohit Thrust (Nandy, 1973). North of MCT in theSiang Valley, where lies the study areas, the NW-SE trendingTuting Fault within the kyanite-sillimanite bearing garnet-biotite schist and gneiss of the Sela Group extends over adistance of about 12 km following the straight course of theriver Siang.

Geomorphic setup at Tuting shows the development offour levels of terraces (Fig 2a). The oldest fluvial terrace, theT4, lies at 600 m above mean sea level (amsl), the T3 at 550 mamsl. The FT2 is made up of a fan that emerges from T3 andpartly aprons the subsequent younger terrace T1. The T1 islocated at 460 m amsl. Thick sedimentary cover composedof fluvial gravels and sand underlain by a bedrock stepscharacterizes these terraces. The samples for OpticallyStimulated Luminescence (OSL) dating were collected fromthe sandy layers of the terrace sequences (Fig 2b). Theterrace configuration of these terraces indicates that thebedrock step lie at ~10 m, ~15 m and 24 m below the T4, T3and T1 respectively.

METHODOLOGY

The alluvial cover lying over the bedrock steps weresampled in opaque stainless pipes. The quartz and feldsparfractions were extracted by treating the samples sequentiallywith HCl, H2O2 and heavy liquid separation using Sodiumpolytungstate (density=2.58 g/cm3). These grains were thensieved to get 90-150 µm size range and the quartz separateswas further etched using 40% HF for 80 minutes followedby 12N HCl treatment for 40 minutes to remove anycontribution from alpha irradiation. The purity of quartzextract vis-à-vis feldspar was tested using Infrared StimulatedLuminescence (IRSL). The grains were mounted on stainless-steel disks using Silko-Spray silicone oil. Luminescencemeasurements were made on a Riso TL/OSL-12 equippedwith IR Diodes emitting at 875 nm (for feldspar) and blue

diodes emitting at the wavelength of 470 nm (for quartz).The detection optics comprised a standard combination ofBG-39+U-340 optical filters mounted on an EMI 9635QAphotomultiplier tube. A 90Sr/90Y beta source-delivering doseat the rate of 0.1 Gy/s was used for irradiation.

The equivalent dose (ED) measurement on quartz wascarried out using a 5-point SAR protocol of Murray andWintle (2000). A pre-heat of 220 °C/10 s for natural andregeneration doses was used. The OSL was measured at125 °C and the ED analysis was confined to aliquots withrecycling ratio of 1±0.1. The initial 2 channels out of 100 ofa shine down curve were used for analysis where eachchannel comprised 0.4 s of shine down.

ED estimation on Feldspar separate utilized a preheattreatment of 250 °C for 60s, to both the dose and test dosemeasurements (Huot and Lamothe, 2003; Blair et al. 2005).Subsequently, a first routine IRSL stimulation was carriedout at 50 °C for 100 s, followed by an elevated temperatureIRSL for 100 s at 225 °C. After every SAR cycle, the samplewas subjected to 40s IR stimulation at 290 °C to reduce

Fig.2. (a) Photograph showing the four levels of terraces at Tuting.(b) Cross section across Siang river at Tuting showing thegeomorphic configuration of the terraces. Note the Quartzluminescence ages of the terraces.



recuperation (Buylaert et al. 2007, 2009; Wallinga et al. 2007;Table 1). For all calculations (both IR at 50 °C and elevatedtemperature post-IR IR) the initial 2 channels of the post-IRIR signal minus a background derived from the last 10channels was used.

RESULTS AND DISCUSSION

Table 2 provides the details on dosimetery of the six

samples. Palaeodose and age estimates yielded by quartzusing SAR at 125 °C are given the Table 2. Table 3 givesdetails of Feldspar analysis both at 50 °C and at an elevatedtemperature of 225 °C. Figure 3 shows an example of shinedown curve, growth curve and dose distribution of analysisdone on quartz, routine and elevated temperature feldspar.Table 4 provides age data on quartz. Single AliquotRegeneration (SAR) analysis based on terrace samples thatin most cases yielded a wide palaeodose distributionindicating inhomogeneous luminescence bleaching ofsediments. This implied that the lower palaeodoses thatrepresent the most bleached part of the sediment to beconsidered for final age calculations (Preusser et al. 2009;Olley et al. 1998; Srivastava et al. 2006, 2009). Similarly,usage of small sub-samples (aliquots) increases thepossibility of detecting poorly bleached grains (Wallinga,2002). However, the sediment samples dated from Himalayanrivers suggests a general problem of low luminescencesensitivity of quartz grains, which limits the possibility ofreducing the aliquot size, and also the poor photon countsputs constraints on the number of aliquots that can be used

Table 1. Dating protocol used for De estimation at elevated tempera-ture of feldspar

1) Dose

2) Preheat (250 °C for 60 s)3) IRSL, 100s at 50 °C

4) IRSL, 100s at 225 °C Lx

5) Test dose

6) Preheat (250 °C for 60 s)7) IRSL, 100s at 50 °C

8) IRSL, 100s at 225 °C T x

9) IRSL, 40s at 290 °C

10) Return to step 1

Table 2. Dosimetery and dose rate data of the samples

Lab. no. Sample Location U T h K Dose rate (Gy/ka)

Latitude Longitude (ppm) (ppm) (%) Quartz Feldspar

LD-517 TS-1 N29° 00’ 21.1" E94° 53’ 17.6" 3.3 23.1 2.7 4.5±0.4 5±0.5LD-518 TS-2 N28° 59’ 37.2" E94° 53’ 39.5" 1.7 21.3 2.3 3.7±0.3 4±0.4LD-519 TS-3 N28° 59’ 37.2" E94° 53’ 39.5" 2.8 20.2 2.6 4.4±0.4 5±0.4LD-520 TS-4 N28° 59’ 50.0" E94° 53’ 43.8" 2.3 14.2 2.5 3.5±0.3 4±0.4LD-521 TS-5 N28° 58’ 53.3" E94° 54’ 10.1" 2.0 16.6 2.8 4.0±0.4 4±0.4LD-522 TS-6 N29° 02’ 29.5" E94° 53’ 54.9" 2.2 18.3 2.7 4.0±0.4 4±0.4

Table 4. Palaeodose and ages estimated on Quartz

Lab. no. Sample Geomorphic Dose rate Paleodose (Gy) Age (ka)

feature (Gy/ka) Mean Least Mean Least

LD-517 TS-1 Terrace T4 4.5±0.4 150±60 92±4 34±14 21±2LD-518 TS-2 Terrace T3 3.7±0.3 89±34 41±1 24±9 11±1LD-519 TS-3 Terrace T3 4.4±0.4 95±31 57±6 23±8 14±2LD-520 TS-4 Fan Terrace T2 3.5±0.3 58±23 26±1 16±7 8±1LD-521 TS-5 Terrace T1 4.0±0.4 69±26 31±7 18±7 8±2LD-522 TS-6 Valley fill ~6 km 4.0±0.4 68±14 54±9 17±4 14±3

upstream of Tuting

Least consists the least 30% of the palaeodoses. Water content assumed to be 15±5% by weight

Table 3. Palaeodose and ages estimated on Feldspar. Note the difference in ages between routine and elevated temperature feldspar

Palaeodose at Age at

Lab. no. Sample Geomorphic Dose rate 50oC 225oC 50oC 225oCfeature (Gy/ka) Mean Least Mean Least Mean Least Mean Least

LD-517 TS-1 Terrace T4 5±0.5 120±21 74±5 185±30 112±3 24±5 15±2 37±7 23±2LD-518 TS-2 Terrace T3 4±0.4 121±12 102±4 157±32 119±13 29±4 24±3 38±8 28±4LD-519 TS-3 Terrace T3 5±0.4 134±27 100±2 242±34 175±6 29±6 22±2 53±9 38±4LD-520 TS-4 Fan Terrace T2 4±0.4 181±19 139±3 114±19 85±1 45±6 35±3 28±5 21±2LD-521 TS-5 Terrace T1 4±0.4 124±31 76±5 — — 28±7 17±2 — —LD-522 TS-6 Valley fill ~6 km 4±0.4 139±28 102±11 — — 31±7 23±3 — —

upstream of Tuting



meaningfully in age estimation (Jaiswal et al. 2008; Ray andSrivastava, 2010). In case of Feldspar smaller aliquot couldhave been used but for the purpose of comparison weavoided it to maintain the similarity in protocols. Feldsparcontamination in quartz is another problem as this mineral,being brighter in terms of luminescence emission, masksthe quartz signal. Infrared stimulated luminescence (IRSL)performed on the sample tends to reduce the feldspar signalto acceptable limits, approaching the quartz signal onlymarginally (Jain and Singhvi, 2001). In case of clean quartzsample (no feldspar contamination) the IRSL output will belimited to 100-150 photon counts.

Therefore, from here onwards quartz analysis means theanalysis done of feldspar free Quartz where we have usedleast 20% of the ages. Sample collected from 11 m below thesurface of T4 terrace yielded a Quartz age of 21±2 ka (LD-517). The two samples collected from near the bottom andtop of the alluvial cover comprising the terrace T3 yielded

quartz ages of 14±2 ka (LD-518) and 11±1 ka (LD-519)respectively. Sample collected from a section from a 12 mthick sequence yielded a quartz age of 8±1 ka (LD-520) andthat collected from 5 m below the top of T1 terrace alsoyielded 8±1 ka (LD-521). A fill sequence located at ~6 kmupstream of Tuting yielded an age of 14±3 ka (Fig. 2b).

Ages on feldspar following the routine and ET protocolare given in Table 3. In general these ages, when comparedto quartz, are overestimated by a factor of more than twoexcept in LD-517 where routine and ET Feldspar ages are15±2 ka and 23±2 ka respectively.

The comparison of Quartz and feldspar chronologiesindicates that: (1) the palaeodose of respective samples areleast in quartz followed by routine feldspar followed by ETfeldspar, (2) In terms of luminescence bleaching, the feldspar,both routine and ET, signals suggest incomplete bleaching,(3) In Himalayan setup, such as Tuting, the extent ofincomplete bleaching seems overtaking the anomalous

Fig.3. The shine down curve, growth curve, and dose distribution of the equivalent doses (De’s) of sample LD-520 (a) Quartz (b) routinefeldspar (c) Elevated Temperature Feldspar.

Time (s)0.0 10.0 20.0 30.0 40.00

1000

2000

3000

4000

OSL Record: 33

Palaeodose0.0 400.0 800.0 1200.0

0

2

4

6

8FrequencyN = 14 Mean = 664.6± 237.50

Dose (s) / 1000.0 2.0 4.0 6.0 8.0

0.0

1.0

2.0

3.0

Lx/Tx

Time (s)0.0 40.0 80.00

40000

80000

120000

160000IRSL Record: 30

Palaeodose800.0 1200.0 1600.0 2000.0

0

4

8

FrequencyN = 25 Mean = 1152.8± 159.62

Dose (s) / 10000.0 0.5 1.0 1.5 2.0

0.0

0.4

0.8

1.2

1.6

Lx/Tx / 10

(A)

(B)

Time (s)0.0 40.0 80.00

20000

40000

IRSL Record: 59

Palaeodose1000.0 2000.0 3000.0 4000.0

0

4

8

12

FrequencyN = 30 Mean = 2232.6± 414.76

Dose (s) / 10000.5 1.0 1.5 2.0

0.0

1.0

2.0

3.0

4.0

Lx/Tx / 10

(C)



fading in feldspar and hence any chronology developed onfeldspar may require an independent test of validity. Theresults given here are although site specific but potentiallyput a caution on situations where dating of feldspar ispreferred on quartz due to (1) the poor luminescencesensitivity in quartz (Srivastava et al. 2008 in Alakanadavalley; Ganga River exit; Sinha et al. 2010; Dutta et al.(2012); Phartiyal et al. 2009 in the Spiti valley, NW Himalaya;Srivastava et al. 2009 at Zero in the Subansiri valley,Arunachal Pradesh, NE Himalaya) and (2) excessive feldsparcontamination (e.g. see Ray and Srivastava, 2010).

The quartz owing to its better bleaching responses isalways preferred over feldspar. Therefore for furtherdiscussions we use ages derived on Quartz. The agessuggest a phase of aggradation ~21 ka (T4) and between14-11 ka (T3) and then ~8 ka (T1). The fan terrace (FT2) sitson terrace T1 yielded an age of 8±1 ka but is morphostrati-graphically younger. Therefore considering the associatederrors we assign the age of fan terrace to be ~7 ka. Thisindicates a nearly continued valley aggradation from>21-8 ka with three phases of bedrock incision. Similar agesof aggradation are also reported from the exit of BrahmaputraRiver at Pasighat (Srivastava et al. 2009) and from theAlaknanda-Ganga (Ray and Srivastava, 2010; Juyal et al.2010; Sinha et al. 2010; Dutta et al. 2012); Spiti river valley(Phartiyal et al. 2009), NW Himalaya and this phase ofaggradation coincide with a climatic transition from coldand dry Last Glacial phase to warm and wet HoloceneOptimum (Prell and Kutzbach, 1987). Thus themorphostratigraphic setup at Tuting seems mimicking theregional fluvial responses to global climate change acrossthe Himalaya but in order to reveal better picture a moredetail study of whole Brahmaputra valley is warranted.

Valley aggradation history, which as a climate responsehas to be regional in nature. However, bedrock incision maylocalized that if studied has potential to unravel riverresponse to neotectonic uplift along a local fault (Srivastavaand Misra, 2008). This approach suggests that the thicknessof the incised bedrock equals to the uplift. At Tuting, first

phase of bedrock incision from T4 to T3 took place <21 kaand from T3 to T1 ~11 ka and that from T1 to present dayriverbed, the T0 took place ~8 ka. The fact that the bedrockunderlying these terraces is covered with thick alluvium,these terraces cannot be used to estimate the uplift rates(for details see Ray and Srivastava, 2010; Srivastava andMisra, 2008). These three incision events thus indicatetowards major episodes of tectonic uplift in the region.

CONCLUSIONS

This investigation offers following conclusions:1. The chronological analysis on quartz and feldspar on

the terrace sediments of Siang River, NE Himalayaindicates that both routine and elevated temperatureprotocols of feldspar dating yield ages that are higherthan those derived using quartz.

2. The overestimation of feldspar ages suggests towardsincomplete bleaching of feldspar in the givendepositional settings on Siang River, at Tuting, NEHimalaya. This has actually overtaken the effect ofanomalous fading of the luminescence signal.

3. The quartz chronology indicates a nearly continuedvalley aggradation from >21-8 ka with three phases ofbedrock incision coinciding with a climatic transitionfrom cold and dry Last Glacial to warm and wet HoloceneOptimum. This phase of valley aggradation is alsoreported from the NW Himalaya.

4. The valley aggradation is punctuated by three phasesof bedrock incision at <21 ka, ~11 ka and at ~8 ka.These three incision events indicate major episodes oftectonic uplift in the region.

Acknowledgements: The Director, Wadia Institute ofHimalayan Geology, Dehradun is acknowledged for thenecessary facilities. Ms Swati Chauhan and Ms Preeti Joshianalyzed the samples. Comments from an anonymousreviewer helped in improving the manuscript.

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(Received: 31 May 2010; Revised form accepted: 25 July 2011)

optically stimulated luminescence chronology of terrace sediments of siang river, higher ne...

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