2007 suresh et al, sedimentology

Evolution of Quaternary alluvial fans and terraces inthe intramontane Pinjaur Dun, Sub-Himalaya, NW India:interaction between tectonics and climate change

NARAYANAPANICKER SURESH, TRILOKI N. BAGATI, ROHTASH KUMAR and VIKRAMC. THAKURSedimentology Group, Wadia Institute of Himalayan Geology, 33, General Mahadev Singh Road, DehraDun 248 001, India (E-mail: [email protected])

ABSTRACT

Quaternary alluvial fans in the tectonically active Pinjaur Dun, an

intramontane valley in the Sub-Himalaya, were deposited in front of the

Nalagarh Thrust and were influenced both by tectonics and glacial climate

fluctuations. The surface morphology indicates that an earlier set of first-order

fans (Qf1) became entrenched and onlapped by a series of second-order fans

(Qf2). The younger fan segments were then cut by a pair of terraces (T1 and

T2). Quartz optically stimulated luminescence dating establishes that the Qf1

aggradational phase was initiated before 965 253 ka and terminated after837 163 ka. This was followed by a period of incision, before Qf2 fandeposition started at 724 134 ka and continued until 245 49 ka.Sediment was deposited on the T1 (upper) and T2 (lower) terraces at

163 21 and 45 ka, respectively, recording a return to overall degradationpunctuated by minor deposition on terraces. The period of incision separating

the younger and older fan deposits coincided with enhanced SW monsoon

precipitation. The subsequent development of the Qf2 fans and their

progradation until 20 ka suggest erosional unloading of the thrust

hangingwall during a tectonically quiescent phase. Toe cutting, deposition

of axial river and lacustrine facies, and retreat of Qf2 around 45 ka, indicate

fanward shift of the axial river due to tilting of the valley towards the NE in

response to reactivation of the Nalagarh Thrust. The cessation of Qf2

deposition around 20 ka and the onset of through-fan entrenchment suggest

reduced sediment supply but relatively high stream power during the last

glacial maxima (LGM). The prolonged stream incision since the cessation of

Qf2 deposition, with only minor depositional phases at 163 21 and 45 ka,resulted from high water discharge and low sediment input during

intensification of the SW monsoon and vegetation changes in the hinterland.

Keywords Optically stimulated luminescence dating, Pinjaur Dun, Quater-nary alluvial fans, Sub-Himalaya, tectonics and climate, terraces.

INTRODUCTION

Alluvial fans are prominent geomorphologicaland depositional features in mountain-front set-tings. They often exhibit alternate phases ofaggradation and entrenchment caused by tec-tonic, climate and/or base-level changes (Bull,1977; Harvey, 1984). Tectonics often determinethe sediment load and the relief at the mountain-

front, promoting fan aggradation (Davis, 1905;Blissenbach, 1954; Bull, 1964; Denny, 1967;Hooke, 1967; DeCelles et al., 1991; Hartley,1993; Viseras et al., 2003), whereas climate playsan important role in setting the rate of sedimenttransport, relief equilibrium and the fan geometry(Lustig, 1965; Harvey, 1990, 1996; Bull, 1991;Dorn, 1994; Ritter et al., 1995; Roberts, 1995).Sediment supply and the rate of catchment

Sedimentology (2007) 54, 809833 doi: 10.1111/j.1365-3091.2007.00861.x

2007 The Authors. Journal compilation 2007 International Association of Sedimentologists 809

erosion are also related to stream power, which isgenerally a function of precipitation intensity.Independent tectonic and climate signals are thusboth involved in many fan sequences and thechallenge is to separate and understand therelative impact of each (Harvey, 2002; Viseraset al., 2003). Late Pleistocene to Holocene alluvialfans in the Sub-Himalaya (Fig. 1A and B) are anideal place to address this problem as this regionis tectonically active and has undergone signifi-cant late glacial climate change (Nakata, 1972;Ruddiman et al., 1989; Valdiya, 1992, 1993;Singh et al., 2001; Goodbred, 2003; Srivastavaet al., 2003; Gibling et al., 2005; Sinha et al.,2005).The Sub-Himalaya, bordered by the Main

Boundary Thrust (MBT) in the north and theHimalayan Frontal Thrust (HFT) to the south(Fig. 1B), is characterized by a series of longitud-inal intramontane valleys known as duns (Nos-sin, 1971). The prominent duns in the NWSub-Himalaya are Soan Dun, Pinjaur Dun andDehra Dun (Fig. 1B). These duns were formed as aconsequence of tectonic activity along the HFT inthe Middle Pleistocene (Raiverman, 2002) thatterminated Siwalik sedimentation in the forelandaround 02 Ma (Ranga Rao et al., 1988). Sedimentdeposited since the Middle Pleistocene has beenmostly accommodated in the duns as alluvial fansand river terraces (Nakata, 1972). The records offan deposition and fluvial terrace formation andtheir significance in terms of tectonics andclimate are poorly understood. Preliminaryquartz luminescence ages for the deposits of twofans in Pinjaur Dun indicate that alluvial fandeposition was initiated well before 57 ka andceased around 20 ka (Suresh et al., 2002).In this study, the evolution of the Quaternary

alluvial fan and terrace deposits in the PinjaurDun (Fig. 1B) is addressed using a comprehensiveset of optically stimulated luminescence (OSL)dates combined with detailed sedimentologicalstudy of the deposits. The new chronology is usedto assess the roles of thrust tectonics and climatechange in controlling episodes of fan growth anderosional degradation along the Himalayan front.

REGIONAL GEOLOGY AND DRAINAGE

The intramontane valley extending between Pin-jaur town in the east and Una town in the west isknown as PinjaurSoan Dun (Fig. 1B). The dun iselongated in a NWSE direction and is about140 km in length. The valley is narrower

(10 km) in the south-east between Pinjaur andNalagarh, where it is known as Pinjaur Dun,widening to 19 km in the north-west, betweenNalagarh and Una, to become Soan Dun and theSatluj Valley. The present study covers the areabetween Pinjaur in the eastern sector and Kirat-pur in the western sector of the valley (Figs 1Band 2A,B).The Pinjaur Dun (750280 m above mean sea-

level, amsl) is bordered by mountains (13001800 m amsl) to the north along the NalagarhThrust (NT), an imbricate branch of the MBT(Figs 1B and 2B). Bedrock in the mountainscomprises the Cenozoic Subathu, and Dharamsalaformations and the Siwalik Group. The Subathuand Dharamsala formations comprise reddish-brown shale, greenish-grey and reddish sand-stone and minor limestone, whereas the SiwalikGroup consists of brown mudstone, grey sand-stone and conglomerate. The south-western mar-gin of the dun is delimited by the Siwalik hills(621 m amsl), which are separated from Indo-Gangetic Plain (400 m amsl) by the HFT. TheHFT is segmented by various NS trending trans-verse faults (Fig. 1B), is currently the most activethrust and has displaced Quaternary deposits(Nakata, 1972, 1989; Valdiya, 1993; Lave &Avouac, 2000; Kumar et al., 2002).The longitudinal rivers in the PinjaurSoan

Dun are the Satluj and Ghaggar rivers which arepart of the Indus drainage system. These riversjoin the Indo-Gangetic Plains through gorgesalong the transverse faults that displace theSiwalik hills (Figs 1B and 2A,B). The major partof the dun is controlled by the Satluj drainagesystem, which consists of two converging axialrivers, the Satluj and Sirsa rivers, and numeroustransverse tributary streams (locally known askhad, nadi, nala or choa). The Satluj River is aperennial antecedent river originating from gla-ciers in the Trans Himalaya, it cuts across theHigher and Lesser Himalayas and flows along thevalley axis. Ephemeral transverse streams havetheir major discharges during the SW monsoonseason between July and September and can bedivided into three categories, based on theirsource in either the mountains to the north, thenorthern flank of the detached Siwalik ranges orthe intramontane valley itself. The streamsdraining the mountains north of the NT havelarger drainage basin areas (Table 1) and widerstreams. The streams draining the detachedSiwalik ranges have relatively small drainagebasins, steep gradients and are dry throughoutthe year except for a few hours immediately after

810 N. Suresh et al.

2007 The Authors. Journal compilation 2007 International Association of Sedimentologists, Sedimentology, 54, 809833

the monsoonal rains. The third category ofstream originates within the valley itself, andthese drain the adjacent alluvial fans and inter-fan areas. The axial Sirsa River in the south-eastern part of the dun flows close to thedetached Siwalik ranges (Fig. 2A and B). Trans-verse alluvial fans and depositional fluvial terra-

ces within both the south-eastern and north-western sectors of the dun occur at multiplelevels (Fig. 3C; Khan, 1970; Nakata, 1972; Karir,1985). In the south-east, Nakata (1972) recog-nized an older and higher Kalka surface and ayounger and lower Pinjaur surface associatedwith the alluvial fans, with the latter cut by the

B A0 500 km

MCT

MBT

Pamir

Indian Shield

TIBET

N

Trans-HimalayaOphiolitic SutureTethys Himalaya

Higher HimalayaLesser Himalaya

Sub-HimalayaIndo-Gangetic Plain

Ganga R

HFT

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s R

Brah

map

utra

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MCT - Main Central ThrustMBT - Main Boundary ThrustHFT - Himalayan Frontal Thrust

INDIA

Study Area

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km0 20 40 60

Siwalik groupDharmsala Fm.

Subathu Fm.Lesser HimalayanMetasediments

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Ganga R

.

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HARDWARIndo-Gangetic Plain

PINJAUR

Beas R

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.

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j R.

FF Transverse Fault

Synclinal AxisAnticlinal Axis

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Fig. 1. (A) Regional geological map of the Himalaya showing tectonic sub-divisions. (B) Geological map of a part ofNW Sub-Himalaya (after Powers et al., 1998) showing various duns. The Pinjaur Dun is bordered by the detachedSiwalik ranges to the south-west and Tertiary mountains in the north-east.

Quaternary alluvial fans of sub-Himalaya, India 811


Jhajulla (or Jhajra) fluvial terraces and the recentalluvial plains. The Quaternary deposits areoverthrust by Tertiary rocks along the NT(Fig. 3A) and in places small patches of theformer occur at higher elevation above the allu-vial fans of the Pinjaur Dun in the hangingwall(Fig. 3B).

METHODS

The sedimentology of the Quaternary depositswas documented using vertical logs from exposedcliff sections on the banks of the transverseflowing entrenched streams, along roads and inirrigation canal cuts. These sections provide both

longitudinal transects from fan apex to fan toeand valley parallel exposures illustrating thelateral and vertical facies trends. The lithofacieswere grouped into gravel, sand and mud categor-ies and further sub-divided into sub-facies basedon grain-size, internal organization, stratification,colour and bedding geometry (Table 2). In thecase of the gravel facies, the maximum, mean andmodal size of the clasts, the matrixclast ratio, thenature of the matrix, sedimentary structures andclast compositions were recorded. For finer-grained lithologies, the percentages of sand, siltand clay were estimated using grain-size analysisfollowing Galehouse (1971). The fan profiles areconstructed using Survey of India topographicalmaps (1:50 000 scale) with 20 m contour intervals.

Table 1. Summary of alluvial fans in the western and eastern sectors of the Pinjaur Dun.

Name ofalluvial fan

Fan dimensions(length width)

Drainagebasinarea (km2)

Alluvialfan area(km2) Remarks

Western sectorLuhund 12 10 km 38 87 Fan is entrenched from apex to toe by streams

(LuhundMisewal Khads) debouching from theTertiary mountains. The fan margins areentrenched by interfan streams

Kundlu 16 6 km 22 67 Fan is entrenched from apex to toe by a stream(Kundlu Ki Khad) debouching from the Tertiarymountains. The fan margins are entrenched by aninterfan stream on the western side and a Tertiarymountain stream leaving on the eastern side

Chikkni 11 7 km 20 44 Fan western margin is entrenched by a streamdraining the Tertiary mountains whereas the easternmargin is entrenched by an interfan stream(Chikkni Khad). Small patch of relict LowerSiwalik rocks are exposed above the fan surface

Eastern sectorRatta 5 3 km 25 11 Fan eastern margin entrenched by a stream sourced

in the Tertiary mountains (Ratta Nadi) whereas thewestern margin is entrenched by an interfan stream(Phula Nala)

Balad 725 375 km 95 32 Fan margins are entrenched by streams draining theTertiary mountains, Balad and Ratta Nadis on theeast and west respectively

Nanakpur 625 61 km 42 36 A group of highly coalescing fans, dissected byNanakpur and Ramnagar Nadis and Surajpur Choa

Kiratpur 725 48 km 17 32 Fan is entrenched from apex to toe by Kiratpur Nadi.Fan western margin is entrenched by a streamdraining from the Tertiary mountains(Ramnagar Nadi) whereas the eastern margin isentrenched by an interfan stream (Sirsa Nadi)

Jhajra 10 8 km 10 41 Fan is entrenched from apex to toe by Jhajra Nadi(Ghaggar drainage system). Fan eastern margin isentrenched by a stream draining from the TertiaryMountain (Koshallia Nadi) whereas the westernmargin is entrenched by an interfan stream(Sirsa Nadi)



The chronology of the fan and terrace deposits(Table 3) was established using optically stimu-lated luminescence (OSL) at the Wadia Instituteof Himalayan Geology, Dehra Dun, India. Sixteensamples of fine-grained to medium-grained sand

from different fan and terrace locations werecollected using metal pipes (25 35 cm). Quartzgrains (90125 lm size) were extracted from thebulk sample after removing carbonate and organicmatter using 1 n HCl and 30% H2O2, respectively,

AB

Fig. 2. (A) Alluvial fans in the north-west and south-east sectors of Pinjaur Dun. (B) Geomorphological map of thePinjaur Dun showing the drainage system and distribution of fan and terrace surfaces. The older fan surfaces havesteeper slopes than the younger fans (contour lines at 20 m interval). The fan boundaries are shown as dotted lines.



and were then sieved and separated using sodiumpolytungstate solution under subdued red lightconditions. The extracted quartz grains wereetched for 60 min in hydrofluoric acid (48%) toremove the outer 10 lm layer (Aitken, 1985),washed with HCl and distilled water and re-sieved. The equivalent dose (ED) was determinedby the multiple aliquot additive dose method in

an automated Riso TL/DA 15 reader (RisoNational Laboratory, Roskilde, Denmark) equip-ped with filtered green light from a halogen lamp.Between 24 and 48 aliquots were prepared persample and each aliquot was exposed to shortduration (03 sec) green light stimulation in orderto obtain values for short shine natural normal-ization (Smith et al., 1986). Each aliquot waspreheated to 220 C for 5 min and both thenatural, as well as beta irradiated (calibrated Sr/Y90 beta source) luminescence, were measured toobtain the additive growth curve. The ED valueswere calculated (by Grun software; Rainer GrunRadiocarbon Dating Research Unit, RSPacS, Can-berra, Australia) using the short shine normalizedtotal integral of the OSL over 20 sec. For theannual dose rate estimation, concentrations ofuranium and thorium in the sediments werecalculated by thick source alpha counting (Ait-ken, 1985) using a Day Break 583 series alphacounter (Day Break Nuclear and Medical SystemsInc., Guildford, CT, USA). The potassium con-centration was measured by X-ray fluorescence(XRF). The cosmic dose contribution was notcalculated and an average value (150 lGy a)1)was used. Water content was determined for allthe samples by heating at 100 C (Table 3). Rep-resentative shine down curves, ED as a functionof OSL measurement time (shine plateau) and thegrowth curve using integrated counts within thefirst 20 sec of OSL measurement are shown inFig. 4.

GEOMORPHOLOGY OF THEINTRAMONTANE VALLEY

The main geomorphological units in the PinjaurDun are shown in Fig. 2. A series of elongate toconical, transverse alluvial fans are located alongthe NE margin of the dun (Fig. 2), and these aresegmented and entrenched (Fig. 3C) throughouttheir length by streams draining from the hang-ingwall mountains NE of the NT. Smaller streamssourced on the fans themselves also are en-trenched. No similar transverse fans are presentalong the SW side of the dun. The entrenchedtransverse streams, and the axial trunk rivers withwhich they connect, are flanked on either side bytwo laterally extensive terrace surfaces (Fig. 3C).

Alluvial fans

Eight alluvial fans, three in the north-west andfive in the south-east parts of the dun are

B

Quaternary deposit

Tertiary Rocks

Tertiary Rocks

Ratta Fan head trench

Nalagarh ThrustA

T2 TerraceQf2Qf1

C

Fig. 3. (A) View of the apex of the Ratta Fan withhinterland Tertiary rocks in the hangingwall of theNalagarh Thrust. (B) Quaternary deposits underlain byTertiary rocks in the hangingwall of Nalagarh Thrustabout 2 km NE of Dehni village. These deposits occur ata higher elevation than the alluvial fans of Pinjaur Dun.(C) Multi-level geomorphological surfaces in the south-eastern part of Pinjaur Dun, upstream of Sirsa Nadi.



Table 2. Summary of the facies identified in this study.

Facies Sub-facies Description Interpretation

Gravels Disorganized,matrix-supportedgravel (facies G1)

Pebble-boulder clasts, chaotic fabric,sub-angular to sub-rounded, verypoorly sorted, muddy matrixsupport, crude stratificationobserved locally. Beds have sharp,non-erosional basal contact. Gravelcontent range from 30% to 60%(Fig. 6A)

Poorly sorted, muddy matrix-supported gravel and chaotic fabricsuggests deposition by cohesiveclast-rich debris flow. Presence ofcrude stratified gravel suggestsremobilization of debris flow bywater action

Disorganized, matrixto clast-supportedgravel (facies G2)

Pebble-boulder clasts, poorly sorted,disorganized, ungraded, matrix toclast support, outsize clasts up to 1 mare common, clasts are sub-roundedto rounded, coarse sand to siltymatrix. Beds have irregular, planarbasal contact (Fig. 6B and C)

Poor sorting, polymodal clastdistribution and absence ofstratification and chaotic fabricsuggests rapid deposition by hyper-concentrated flood flow duringcatastrophic flood event and/orheavy rainfall/cloud burst

Crudely stratifiedgravel (facies G3)

Pebble-boulder clasts, poorly tomoderately sorted, crude horizontalstratification, well-developedtransverse clast fabric, matrix to clastsupport, clasts are well- tosub-rounded, planar to erosionalbasal contact. Ungraded to normalgrading (Fig. 6B,C and D)

Crude stratification and preferredclast fabric suggest deposition bypersistent steam flows. Thesecharacteristics are common ingravels transported as bed load anddeposited under waning flow byaccretion of progressively smallerclasts, in channels and onlongitudinal bars

Cross-stratifiedgravel (facies G4)

Well-organized, clast- to matrix-support, pebble-cobble clasts. Lowangle cross-stratification (range from10 to 15 but up to 25). Scouringbasal contact (Fig. 6D). Both troughand planar cross-stratifications arepresent. Set thickness is 2550 cm.

Lateral accretion and slip face depositon longitudinal bar or hollow fill

Sand Stratified tomassive sand(facies S1)

Medium- to coarse-grained grey sand,essentially massive, locallycross-stratified, grading normal.Lenticular to sheet geometry(Fig. 6B). Basal contact is sharp,fining upward. Cross set thickness

identified. These are almost parallel to the pre-sent-day transverse streams with their apices atthe foot of the hangingwall mountains and theirtoes terminating at the axial river floodplain(Table 1, Fig. 2A,B). The alluvial fans connect tothe Satluj drainage system with the exception ofJhajra Fan in the SE, which belongs to theGhaggar system (Fig. 2B). The alluvial fans are

coalescent and dissected by headward-erodingsecondary drainages (Fig. 2B).Longitudinal fan surface profiles show that the

alluvial fans in the SE have higher elevations thanthose in the NW (Fig. 5). The concavity andelevation of each fan gradually falls as the axialriver confluence and valley outlet are ap-proached. The slopes of the fans vary between

0

5000

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Exposure time (sec) 0 4 8 12 16 20 I

nten

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nts/c

hann

el)

Laboratory dose (Gy) 200 0 200 400 600 800 0

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200 0 200 400 600 800Laboratory dose (Gy)

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60

80

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(a.u.

)

Fig. 4. Quartz optically stimulated luminescence shine down curve, shine plateau and growth curve for two rep-resentative alluvial fan samples from the Pinjaur Dun.

Table 3. Quartz optically stimulated luminescence (OSL) age, dose rate and equivalent dose of Pinjaur Dun sedi-ments. The U, Th and potassium values and dry water content used for dose rate calculation are also given.

Sl.no.

Sampleno. U (p.p.m.) Th (p.p.m.) K (%)

Watercontent (%)

Dose rate(Gy ka)1)

Equivalentdose (Gy)

Age(ka BP)

1 BGK-T1 366 107 806 365 175 397 326 036 12866 890 395 512 BGK-T4 208 089 1223 306 142 223 300 032 11369 2836 379 1033 BGK-T5 242 093 1145 318 148 152 310 034 11081 2291 358 844 BK-T1 291 10 1353 343 167 062 362 037 16262 2573 450 855 DK-T1 167 059 894 202 119 569 208 019 20100 4928 965 2536 DK-T4 186 095 1317 325 136 044 301 035 7348 1045 244 457 ASN-T1 159 077 1205 264 135 300 277 027 13644 1655 493 778 ASN-T4 270 07 647 239 118 072 251 026 6161 1049 245 499 LKK-T1 290 0.29 93 09 142 126 295 001 15274 2568 517 8610 LKK-T4 39 039 117 12 155 024 361 029 9925 945 275 3411 KK-T2 52 052 111 11 20 625 404 015 16673 2080 413 5412 KK-T4 34 034 124 12 199 765 364 013 30492 5838 837 16313 CR-T1 33 033 124 12 170 808 301 001 21823 4053 724 13414 T1 23 023 101 10 166 075 319 030 5197 456 163 2115 T2-A 50 050 121 12 185 033 423 028 1684 304 40 0816 T2-B 48 048 73 073 093 434 274 028 1340 278 49 11



07 and 42 (Fig. 5) with evidence for twosegments on fans from both the south-eastern(Balad, Kiratpur and Jhajra fans) and north-west-ern (Luhund, Kundlu and Chikkni fans) areas.

The proximal parts of the steeper fan lobes (Qf1)were dissected by fan head incision and a secondmore extensive phase of fan growth (Qf2) oc-curred below the intersection point, onlapping

Kundlu Fan

Chikkni Fan

A Northwest Sector

Balad Fan

Kiratpur Fan

Jhajra Fan

B Southeast Sector

KmKm

Met

ers

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048200

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NT

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Active stream grade

Active stream grade

Active stream grade

Active stream grade

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TertiaryMountain

TertiaryMountain

TertiaryMountainTertiary

Mountain

TertiaryMountain

Satluj River

Satluj River

Sirsa Nadi

Sirsa Nadi

Jhajra Nadi

Luhund Fan

0481216

Qf1

Qf2

Active stream grade

Inte

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poin

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NT

TertiaryMountainSatluj River

T1T2

Active riverS2

0m

G3

G3 *4.9 1.1Ka+ 10m

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Active riverS2

20m

G3 *4.0 0.8Ka+

28m

0m

T1

Active river 20m

0m11m

M1S2S1 *

16.3 2.1Ka+

Fig. 5. Transverse (NESW) alluvial fan surface and channel profiles in the north-western and south-eastern parts ofthe Pinjaur Dun. Qf1 fans have higher gradients than Qf2 fans. The alluvial fans are at a higher elevation (750380 mabove mean sea-level) in the south-east sector compared with the north-west (520280 m above mean sea-level).Position of the T1 and T2 terraces flanking the active channel are shown for the Luhund Fan, together with opticallystimulated luminescence dates.



the medial zone of the earlier fans. The slopes ofthe Qf1 fans vary between 21 and 42 and theQf2 fans between 07 and 21. The Qf2 fans are1116 km long and 610 km wide in the NW, and510 km long and 38 km wide in the SE(Table 1).The alluvial fans have been completely incised

by transverse streams that are presently deliver-ing their sediment load directly to the axial rivers.The lateral fan margins (inter-fan areas) areincised by streams originating from either thehangingwall mountains or within the valley itself.The depth of incision, by both transverse or axialrivers, is slightly higher in the north-west com-pared with south-east. The Qf1 fans are deeplyincised and the proximal fan surfaces are typic-ally 5080 m above the modern stream beds. Theincision of the Qf2 fans by transverse streams hasresulted in exposures between 10 and 35 m highin the proximal and distal sections of the secon-dary fans in the north-west (e.g. the Luhund Fan),and 8 and 15 m high respectively in the south-east (e.g. the Jhajra and Kiratpur fans). In the SEpart, the Kalka and Pinjaur surfaces (Nakata,1972) are equivalent to the top of Qf1 and Qf2,respectively.

River terraces

Two levels of terraces comprising thin veneers ofgravels and sandy sediments resting on eroded fanremnants are developed below the Qf2 surface.Both sets of terraces are paired, occurring at asimilar elevation on either side of the modernriver valleys, including those draining just theinter-fan surfaces (Fig. 3C). The levels are desig-nated the T1 (upper) and T2 (lower) terracesurfaces. The T1 surface (originally termed theJhajra surface by Nakata (1972) is 300800 m inwidth and 46 m below the Qf2 surface, whereasthe T2 is about 50100 m in width and is 23 mbelow T1.

Axial rivers

The axial Satluj river in the north-west has a15 km wide active channel and occupies afloodplain 1012 km wide between the SiwalikHills and the toes of the transverse fans. The Qf2fans (Luhund and Kundlu) are truncated by theaxial river forming a cliff 2035 m high but thereis no obvious incision on the opposite river bank.The axial Sirsa River has a narrower activechannel (

consist dominantly of G1 with subordinate G2and minor G3 (Fig. 8; Baglehr section-1). Medialfan sections are dominated by G2 and G3 with

minor contributions from G4, S1 and M1 facies(Fig. 7, Tikkri section and Dehni section-1). Dis-tally, Qf1 (Figs 79; Dehni-2, Baglehr-3, Barun-1,

Fig. 6. Examples of the main lithofacies: (A) massive mud (M1) overlain by poorly sorted, disorganized muddymatrix supported gravels (G1) in the proximal part of Qf1 (hammer is 32 cm long). (B) Vertical and lateral amalga-mation of S1, S3, G2 and G3 facies in the medial Qf2 Luhund Fan. (C) Vertical and lateral amalgamation of G2 and G3in the medial Qf2 in Kundlu Fan (hammer is 32 cm long). (D) Distal part of Qf2 showing vertical association ofvarious facies in the Chikkni Fan (hammer is 32 cm long). (E) Distal facies (M2 and M1) of Qf1 is overlain by G3 faciesof Qf2 with erosional contact in stream section cutting the Luhund Fan (pen is 14 cm long). (F) Exotic whitish-greypebbly sand (S3) overlaying by lacustrine deposit (M3) of the axial Satluj River in the toe of the Qf2 Kundlu Fan.



Fig. 7. Measured vertical sections with luminescence ages for the Luhund Fan in the north-western sector of thevalley. Patchy outcrops of distal Qf1 facies are exposed beneath the Qf2 fan deposits (Dehni section-2 and Dabhursection-2). The facies distribution shows a dominance of gravel in the proximal fan to sand-mud-gravel in themedial fan to mud-sand in the distal fan region of Qf2. Palaeoflow directions are shown by arrows on the righthand side of the logs. Magnetic polarity stratigraphy for the Baba Gurditta section shows two excursions (fromSangode et al., 2002).



Nangal-2 and 3 sections) is composed of thick (14 m thick) M1 and M2 units with well-developedconcretions, erosionally overlain by G2 and G3gravels attributed to Qf2 (Fig. 6E). In places,thickly bedded sand (S2) is overlain by Qf2

deposits with an erosional contact (Fig. 7, Dabhursection-2). In the south-eastern part of the valley,the proximal part of Qf1 is exposed near Kalkaand Baddi, and consists of G1 with minor M2(Fig. 10).G1 is overlain byM2with a sharp contact.

Fig. 8. Measured vertical sections with luminescence ages for the Kundlu Fan in the north-western sector of thevalley, showing the distribution of various facies from proximal to distal regions. Arrows at the right-hand side of thelog show palaeoflow direction. Legend as in Fig. 7.



Qf2 fans

NW valleyThe Qf2 fans are well exposed along the trans-verse rivers and downstream changes in litho-facies can be documented from proximal, medialto distal fan zones. The facies distribution within

Qf2 deposits of the north-western area (Luhund,Kundlu and Chikkni fans) is illustrated in Figs 79.

Proximal fan facies. The proximal Qf2 deposits(Dehni-2, 3 and 4, Baglehr-2 and 3, Barun-1 andNagal-1, 2 and 3 sections) are 617 m thick, and

CHIKKNI FAN

NN

Chik

knik

had

Bhatian

Sirsa

nadi

Nangal

Nalaga

rh Thru

stTertiary Rocks

0m

5

M2

M2

M2M1

Nangal Section -1(proximal Qf2)

0m

10

M1

M2

M1

M1

G3

Bhatian Section-2(Distal Qf2)

10

0m

G3

G2

M1

Bhatian Section-1(medial Qf2)

M2

S2

G3

Nangal Section-2(Proximal Qf2)

0m

5

G3

G3

G4S2

M2

M1

Prox

imal

Qf2

Dis

talQ

f1

Dis

tal Q

f1

G3

Nangal Section-3(Proximal Qf2)

0m

5

G3

G3

G4

M2

Prox

imal

Qf2

M1

0 2kmG3

Qf2

Qf1

Fig. 9. Measured vertical sections for the Chikkni Fan, north-western sector, showing distribution of various faciesfrom the proximal to the distal fan. Arrow at the right-hand side of the log shows palaeoflow direction. Legend as inFig. 7.



Fig. 10. Measured vertical sections of the Ratta, Balad, Kiratpur and Jhajra fans in the south-eastern sector withoptically stimulated luminescence age for the basal part of Ratta Fan. These fans are dominated by gravel facies anddo not show any marked change in facies from proximal to distal fan. Arrow at the right-hand side of the log showspalaeoflow direction. Legend as in Fig. 7.



have an erosional contact with underlying mudfacies of Qf1 (Figs 79; Dehni-2, Baglehr-3,Barun-1 and Nagal-2 and 3 sections). The Qf2deposits consist mainly of thin to thickly beddedgravels (lithofacies G2, G3 and G4) interbeddedwith S2 and/or poor to moderately developedpalaeosols (M1 and M2; Fig. 11A). G3 is thedominant facies and in places passes laterallyinto G4. The disorganized sandy matrix-supported to clast-supported G2 (75150 cmthick) and lenticular bodies (100120 cm thick)of S2 are interbedded with G3 with irregularcontacts between them (Fig. 11A). Thickly bed-ded M1 and M2 are observed towards the top ofthe section (Figs 7 and 9; Dehni-3 and Nangal-1sections) and locally are yellowish in colour withpedogenic calcrete. No major depositional phasehas occurred since this pedogenic mud and thismarks the termination of Qf2 deposition. Thepalaeocurrent directions obtained from imbri-cated clasts of G3 vary between 170 and 320.

Medial fan facies. Measured sections (Figs 79;Dabhur-1 and 2, Barun-2 and 3 and Bhatian-1sections) from the medial parts of Qf2 fans showmajor variation from gravel-dominated proximalfan facies to gravel (G3 and locally G1), pebbly- tofine-grained sand (S1, S2, S3) and mud (M1, M2and locally M3) facies arranged in 712 m thickcoarsening-up cycles (Fig. 11B). S1 is the domin-ant facies and is very loose, whereas S2 is minorand weakly indurated by carbonate cements.Pebbly sand (S3; pebble size 16 cm) is rare withlow angle planar cross-stratification (Fig. 7; Dab-hur section-1). The silt-grade to clay-grade M1and M2 are very compact and contain hard layersof carbonate parallel to bedding. The mud andfine sand are thin to thickly bedded (10 cm to2 m), massive with sharp to gradational contacts.Towards the top of the sections (Dabhur-1 and 2and Barun-2 sections; Figs 7 and 8), facies G1 andG3 are thickly bedded (255 m thick) and separ-ated by a 1 m thick unit of S2 (Figs 7, 8 and 11B).These gravel units commenced with pebbly,sandy matrix-supported and crudely stratified(G3) but up section, clast size increases to bouldersize (2100 cm diameter, with mean of 15 cm)and the gravels become more poorly sorted, mudmatrix-supported, and disorganized G1 with out-sized clast (long axes >1 m). The mean palaeoflowobtained by clast imbrication in G3 is towards200. These gravel beds extend for tens of metresin a down-stream direction. Decimeter-thick bedsof pebbly sand facies (S3) occur at a few strati-graphic levels (e.g. Dabhur section-2; Fig. 7) but

are rare. S3 clast size varies from 1 to 6 cm indiameter and the clasts float in a medium-grainedsand matrix.

B

A

S2

S2

G3

G3

G3

C

M1

G3

M1

M1

M1M2

M1S2

M1S2

S1

Fig. 11. (A) Association of G3 and S2 facies in theproximal Qf2 of Ratta fan (height of man standing is16 m). (B) Medial Qf2 (Dabhur section, Luhund Fan)showing the dominance of sand-mud facies capped bygravel facies towards the top of the section (outcropheight is around 20 m). (C) Distal Qf2 (Baba Gurdittasection, Luhund Fan) showing dominance of massivemud with interbedded thinly bedded lenticular gravels(height of man standing is 16 m).



Distal fan facies. The distal parts of Qf2 fans[Bhatoli, Baba Gurditta, Nirmohgarh-1 and 2(Fig. 7), Alowal-1 and 2 (Fig. 8) and Bhatian-2(Fig. 9) sections] are dominated by M1 and rarelyby M2 and M3, with fine-grained to medium-grained S2 overlain by thinly bedded G3 and G4(Fig. 11C). Cross-fan sections parallel to the mainvalley orientation show lateral continuity of bedsup to only a few metres (Fig. 11C). M1 is verycompact, massive and dominantly reddish col-oured. Medium-grained S1 sand is greyish incolour, and very loose whereas the fine-grainedS2 is relatively compact and red to yellowish-brown in colour. Facies G3 and G4 occur atvarious stratigraphic levels (Figs 7 and 8) asthinly bedded lensoid bodies (Fig. 11C). The1 km outcrop between Batoli and Baba Gurudittais characterized by more than five such lensoidgravel bodies with a channel form seen perpen-dicular to the palaeoflow direction. In the BabaGurditta section (Fig. 7), the G3 and G4 gravelunit (pebble to boulders, maximum clastsize 50 cm) at the base of the section is about2 m thick and fills a channel about 20 m wide.However, in the Nirmohgarh sections (Fig. 7) amajor change in facies is observed with S1sandwiched between thickly bedded (up to 5 m)G3, representing a major channel deposit. Sixwell-demarcated stacked channel fills suggestthat the main channel was active at this locationand that the present position of the main streamhas shifted about 1 km towards the west. Thedistal zone of the Chikkni Fan (Bhatian-2 section;Fig. 8) is dominated by thick, massive M1 andthickly bedded fine-grained to medium-grainedS1 and S2. The G3 and G4 gravels are rarelyobserved and consist of pebble-sized clasts(14 cm).

Axial river facies. In the distal part of Qf2, in theAlowal-1 and 2 sections (Fig. 8), white to off-white coloured, medium-grained, micaceoussand (S1, 43 cm to 121 m thick) and alternatingcycles of clay-rich red, grey and yellow mud (M3,082 m thick) are observed NE of the Satluj-SirsaRiver confluence (Fig. 2). The S1 facies rest onG3 gravel belonging to the distal Qf2 fans with anirregular erosional contact, contain mud ballscoated with mica (armoured balls) and pebbles,and are separated by thinly bedded (1 cm) pebblybeds. This sandbody shows large-scale troughcross-stratification and extends laterally (about2 km) up the Satluj River. The mean palaeoflowis towards 95, nearly perpendicular to that ofthe alluvial fans. The overlying facies M3 is

thinly laminated (110 mm layers) with minoroccurrences of thickly bedded (up to 40 cm)grey mud (Alowal-2; Fig. 8). This is overlain bysilt grade reddish mud (M1) and red to yellowbrown sand (S2), similar to that in other distalfan areas.

SE valleyQf2 fans in the south-east of the study area showless obvious internal facies variations from apexto toe; they are dominated by facies G2, G3 andM1 with minor S2 (Fig. 10). The proximal regionof the Ratta Fan (Fig. 10) is dominated by gravels(G3 and G2) and sandy mud (M1). The matrix ofG3 gravels is more silty than those found in thenorth-west. Clast size varies from pebble toboulder grade (240 cm). Facies M1 is thicklybedded (23 m) and is separated by thin layers ofgravel close to the base of the succession. Asimilar facies association is also observed in theproximal part of Qf2 in the Balad Fan. The distalregion of the Balad Fan (Fig. 10) also consistspredominantly of gravel facies (G3 and G4) with alensoid body of fine to medium-sand (S2). Themean palaeoflow obtained by clast imbrication inG3 is towards 250.The Kiratpur Fan does show rapid lateral and

downstream variations from the proximal to thedistal fan. In the proximal region, facies G2 andG3 dominate with thin interbedded M1 mudfacies. However, in the inter-fan area betweenthe Kiratpur and Jhajra fans, the proportion ofmud increases (Fig. 10). Distally, units of mudfacies (M1) up to 8 m thick predominate andthese are sharply overlain by 15 m thick gravel(G3) units with pebble-sized to cobble-sizedclasts. The mud is compact, massive and red incolour. The Jhajra Fan, formed around the JhajraNadi, consists of boulder-sized clasts. In thedistal region of Jhajra Fan (Fig. 10), the domin-ant facies is pebble to boulder grade gravel (G3)with interbedded thin to thickly bedded mudunits.

Terrace faciesThe deposits on the terraces are dominantlymassive, clast-supported to matrix-supported G3facies in beds 0512 m thick showing finingupward cycles and capped by thin (0107 m) S1and/or S2 sands. The sandy facies are occasion-ally overlain by 515 cm thick M1 units. Thelower contact of the terrace facies is erosional andcut into the earlier fan surface, dominantly Qf2,but locally Qf1 near the fan intersection points(Fig. 5, Luhund Fan).



AGE OF THE STRATIGRAPHIC UNITS

Quartz OSL ages for the alluvial fans and terracesof the Pinjaur Dun (Figs 710; Table 3) constrainthe age of the different depositional and inci-sional phases. The oldest dates obtained (965 253 ka) were from a sand unit belonging to thedistal part of the Qf1 Luhund Fan (Dabhursection-2; Fig. 7), and a sandy mud (M2) againfrom Qf1 deposits exposed in the proximalKundlu Fan (Baglehr-3 section; Fig. 8; 837 163 ka).A Qf2 sandy mud in the proximal Ratta Fan

produced an age of 724 134 ka (Fig. 10). In theBatoli section, dating shows the oldest exposedQf2 deposit is 45 85 ka (Fig. 7). A sandy mudrepresenting Qf2 from the proximal part of KundluFan (Baglehr-2 section) gave an age of 413 54 ka(Fig. 8). This unit is from approximately 6 mbelow the upper Qf2 surface (Fig. 8). In the BabaGuruditta section (distal region of Luhund Fan),the oldest Qf2 age measured was 395 51 ka(29 m) with ages of 379 103 ka (1275 m) and358 84 ka (10 m) from the overlying Qf2 sec-tion (Fig. 7). Qf2 sediments from the base of amedial section through the Kundlu Fan (Barun-2and 3 sections) produced an age of 517 86 ka,with an age of 275 34 ka at the top (Fig. 8). Adistal Qf2 section (Alowal-1) produced ages vary-ing between 493 77 ka for the basal part and245 49 ka for the top (Fig. 8). A sand unitoccurring about 10 m from the top of medialLuhund fan (Fig. 7; Dabhur section-2) was datedat 244 45 ka. A palaeomagnetic study carriedout on the Baba Guruditta section indicatestwo Quaternary excursions. The oldest excursionobserved at 40 ka is Lashchamp (details in Sang-ode et al., 2002) and the youngest is Mono Lake(Fig. 7; Baba Guruditta section). OSL dates fordeposition on the fluvial terraces showT1 is 163 21 ka and two ages from T2 are 49 11 ka and40 08 ka.Overall, the new dating shows that Qf1 depos-

ition extended to older than 965 253 ka andcontinued until 837 163 ka. The Qf2 depos-ition phase was initiated at 724 134 ka andcontinued until ca 20 ka (244 45, 245 49and 275 34 ka). Incision of Qf1 occurred after837 163 ka but before 724 134 ka, and inci-sion occurred for Qf2 after sedimentation stoppedat ca 20 ka. The depositional fluvial terraces at163 21 ka (T1 terrace) and 45 ka (T2 terrace;49 11 and 40 08 ka) indicate that the young-er phase of incision was interrupted by minordepositional phases.

DEPOSITIONAL ENVIRONMENT

The alluvial fans in the Pinjaur intramontanevalley were deposited by transverse flowingstreams similar to the present day streamsdebouching from the hangingwall mountains.The radiating pattern of palaeoflow (170300),relatively steep depositional slopes (074), theabsence of fauna, and the rapid decrease in clastsize in a down-fan direction, are characteristicfeatures of alluvial fan deposits. The dominanceof grey-coloured sandstone clasts in the gravelfacies and reddish-coloured sand and mud faciesindicate their derivation from grey-coloured andred-coloured sandstones and mudstones belong-ing to the Dharamsala (Lower Tertiary) and LowerSiwalik (Upper Cenozoic) formations exposed inthe hangingwall (Fig. 1B).A range of sedimentary processes operated

during deposition of the alluvial deposits in thePinjaur Dun. The proximal zones of Qf1 arecharacterized by poorly sorted, disorganized andclay matrix-supported gravels (G1) lacking gra-ding, suggesting deposition by debris flow pro-cesses (e.g. Hubert & Filipov, 1989; Blair &McPherson, 1992; 1994). A high percentage ofclay matrix in the debris flow deposits reflects thesource lithology and suggests that these were highviscosity flows with reduced mobility (Rodine &Johnson, 1976; Pierson, 1981). In medial parts ofthe Qf1 fans, interbedding of facies G2 and G3,overlain by S1, M1 and M2, suggest deposition bysheetfloods with associated hyperconcentratedflood events (e.g. Smith, 1986; Wells & Harvey,1987; Maizels, 1988; Blair, 1999; Gupta, 1999),capped by facies suggesting deposition underwaning flow. In distal fan settings, the presence ofthick bedded M2 facies associated with minor S2facies suggests channel aggradation with the bulkof the silty sediment deposited by overbank sheetflows. The Qf1 fans thus locally reveal a lateraltransition from proximal debris flow to streamflow to distal overbank deposits.The facies associations in the proximal part of

the Qf2 fans resemble those in the medial part ofthe Qf1 fans. Although a minor component, theoccurrence of facies G1 indicates that debris flowsalso occurred locally on the proximal Qf2 fans.The presence of thin beds of sand and mudoverlying G3 are interpreted as waning stagestream flow deposits (e.g. Blair & McPherson,1992). The medial and distal regions of Qf2 fansare dominated by thickly bedded, stratified tomassive sheet sand and mud facies indicative ofa stream flow process with high flood intensity.



The off-white micaceous sand which extendslaterally for 2 km in the distal Kundlu Fan isinterpreted as the deposit of the axial SatlujRiver. The present day axial river course shows asharp bend just south of this location. Theobserved arcuate planform of the distal limit ofthe Luhund and Kundlu fans and the cliffseparating them from the axial river suggestsrecent fan trimming. The sandbody sandwichedin the distal Kundlu fan deposit suggests a periodof fan trimming with deposition of axial riversand above fan deposits dated at 493 77 ka(Alowal-1, Fig. 8). This is consistent with lateralmigration of the axial river (e.g. Leeder & Mack,2001) towards the NE (fanward). The overlyingparallel laminated clays may indicate lacustrinedeposition formed after abandonment of the axialriver.The terrace deposits, T1 and T2, indicate two

periods of incision and subsequent aggradation.The facies associations reveal that the terracedeposits were emplaced gravely bed rivers. Thepresence of extensive paired terraces suggestsdeposition in confined channels in the absence ofmajor tectonic activity in the valley.The source lithology (see Fig. 1B) and inferred

climate were similar between the NW and SEsectors of the valley, but alluvial fan dimensionsand facies varied. Qf1 fans in the NW have smallradii with marked internal facies changes, but aremore elongated in the SE. This suggests that theQf1 fans in the NW developed during a time ofhigher tectonic subsidence along the NT (e.g.Gordon & Heller, 1993; Calvache et al., 1997;Viseras et al., 2003). This resulted in verticalaggradation with higher depositional slopes (4),high sedimentation rates and a lateral transitionfrom debris flow to stream flow. In contrast, in theSE, tectonic subsidence may have been lowerallowing more extensive progradation, lower fangradients (e.g. Viseras et al., 2003) and lessmarked lateral facies trends. The Qf2 fans havesimilar elongate planforms in both sectors, withthe NW fans being longer with extensive downfanlithological variation. Greater valley width(19 km) in the NW allowed the depositionalarea to expand, producing fans with low deposi-tional slopes (071) and differentiated surfacesover which particles were efficiently separated bysize. However, in the south-east, the basin isnarrow (10 km) and hence there was less space,resulting in smaller fans prone to toe trimmingwith higher depositional slopes (121) and adominance of less variable coarser facies fromapex to toe region. The fan evolution was there-

fore sensitive to lateral variations in subsidencerate and receiving basin geometry.Alluvial fans tend to show a strong log-linear

relationship between fan area and the area of theirassociated catchment (Hooke, 1967; Bull, 1977;Harvey, 1987a; Mather et al., 2000). Fan areas inthe south-eastern part of the valley are muchsmaller than expected for their catchment area(Table 1). There are two possible reasons for thisdiscrepancy: (i) a significant reorganization of thedrainage network may have occurred followingdeposition of Qf2 through river capture (e.g.Gupta, 1997); or (ii) as suggested by Bull (1977)the fan area catchment area relationship variesfrom basin to basin and valley to valley. In thiscase the narrow valley to the SE restricts theavailable space for fan deposition.

EVOLUTION OF PINJAURINTRAMONTANE VALLEY

The evolution of the Pinjaur intramontane valleyis summarized in Fig. 12. The valley was createdby tectonic activity on the HFT and associatedfolding. As a result, sedimentation stopped in theSiwalik Basin around 200 ka (Ranga Rao, 1993;Singh et al., 2001). In tectonically active areas,faulting has been emphasized in initiating source-area erosion and deposition of alluvial fans (Davis,1905; Blissenbach, 1954; DeCelles et al., 1991;Hartley, 1993). The deposition of alluvial fans inthe Pinjaur intramontane valley was a conse-quence of tectonic activity along the NT(Fig. 12A). The alluvial fans were deposited un-conformably over the synclinally folded Siwalikrocks in the footwall. The timing of the initiationof tectonic activity along the imbricate thrusts ofthe MBT is not well-known, but conglomerateswith Cenozoic clasts and piedmont fans in theUpper Siwalik Formation suggest its initiationand reactivation at 48 and 177 Ma, respectively(Kumar et al., 1999; Ghosh et al., 2003). Thepresence of similar clasts in the alluvial fans ofPinjaur Dun indicate that the thrust hangingwallmay have continued to supply sediment to thenewly created intramontane valley.Sedimentation on the Qf1 fans continued until

837 163 ka when a period of fan headentrenchment occurred on all the fans along thenorth-eastern margin of the valley. This led todeposition of the secondary Qf2 fan lobes startingat 724 134 ka. Fan entrenchment can resultfrom inactivity on a marginal fault and increasedrates of river down-cutting in the catchment and



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fan apex (e.g. Bull, 1964, 1977), or in response toclimatic change (e.g. White et al., 1996). Harvey(1987b) has suggested that distal trenching in-stead of distal aggradation occurs under condi-tions of reduced sediment supply but relativelyhigh stream power. This suggests that eithertectonic activity along the NT might have ceasedor a more humid climate might have caused thechange to entrenchment of the Qf1 fans. An ice-core oxygen isotope record from the Guliya icecap in the Qinghai Tibetan plateau region(Thompson et al., 1997) provides evidence for astrong summer monsoon influence around 74 ka,consistent with climate influence on fan dynam-ics at this time.The sedimentological data shows that the Qf2

fans have prograded basinward from intersectionpoints on the Qf1 fan surfaces since 724 134 kaproducing coarsening upwards cycles. The pro-gradation may reflect either erosional unloadingdue to cessation of NT activity (e.g. Heller et al.,

1988; Gordon &Heller, 1993) and/or an increase inprecipitation (Fig. 12B). The lower gradient anddominance overall of stream flow-related facies inthe Qf2 fans indicates a high water:sediment ratio.During the last interstadial (5824 ka), the inten-sity of the SW monsoon was enhanced in theHimalaya (Benn &Owen, 1998; Owen et al., 2002).This suggests that the Late Quaternary climatechange may have contributed to and enhanced theentrenchment of the fan deposits.The presence of axial river sediments, lacus-

trine facies and evidence of toe cutting of distalQf2 fans at or after 45 ka indicate that the axialriver migrated towards the fans and becameconfined against them in response to possibletectonic activity on either the HFT or NT(Fig. 12C). However, other evidence for tectonicactivity around 45 ka and time equivalent sedi-ments along HFT are lacking. However, thealluvial fan deposits are overthrust along theNT and uplifted Quaternary gravels are seen

Axial river

Axial river

Axial river

N

Axial river

Axial river

Axial river

Northwestern Sector Southeastern Sector

Qf1

74-45 ka

200-74 ka

45-20 ka

NT

HFT

NT

HFT

NT

HFT

NT

HFT

NT

HFT

NT

HFT

Qf1

Qf1 Qf1

Qf2 Qf2

Qf1

Qf2

Qf1

Qf2

Prior to 200 ka, activity along HFT and formationof intramontane basin.After 200 ka, active phaseof NT, high tectonic subsidence along basinmargin, initiation of Qf1, and substantial verticalaggradation in the western sector.

At around 74 ka, cessation of thrust activity andincision of Qf1. Between 72-45 ka, progradationof Qf2 constricting axial river.

At 45 ka, rejuvenation of NT, accommodationcreated along northeastern basin margin, axialriver shift to fan toes, distally trimming Qf2.Around 40 ka cessation of NT activity,progradation of Qf2 and shifting of axial river tosouthwest during tectonically quiescent phase.At 20 ka cessation of deposition of Qf2.

A

B

C

Fig. 12. Cartoon showing the evolutionary history of alluvial fans and fluvial terraces in the north-western andsouth-eastern sectors of the Pinjaur Dun. HFT Himalayan Frontal Thrust, NT Nalagarh Thrust.



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locally along the hangingwall. This suggests thatmovement along the NT might have causedsubsidence along basin margin and tilting ofvalley towards NE. This correlates well with aregional major tectonic episode in the Himalayaaround 45 ka, when movement occurred on theKaurik-Chango Fault in the lower Spiti valley(Banerjee et al., 1997), the Lamayuru Fault inLadakh (Bagati et al., 1996) and the NainitalFault in the Kumaon Lesser Himalaya (Singhviet al., 1994). Since 45 ka, the axial river sedi-ments and lacustrine facies have been overlainby sand-mud facies associated with the distalQf2 fans indicating further basinward fan growthforcing migration of the axial river away. Thecessation of sedimentation on the Qf2 fansaround 20 ka suggests lower discharge and sedi-ment supply under a more arid climate due toweakening of the south-west monsoon duringthe LGM.

After the cessation of Qf2 deposition, the fansand the axial valley underwent incision, inter-rupted by deposition on a pair of fluvial terraces.The stream incision between 20 to 16 ka en-trenched the Qf2 fans from fan head to toe(Fig. 12D). Prolonged fan entrenchment may haveresulted from lowering of the axial base level and/or a decrease sediment supply. Base-level chan-ges, related to tectonics or climatic change, canresult in fan incision (distal, proximal or throughfan) and have a major influence on fan dynamicsand morphology (Harvey, 2002). The formation ofterraces, with the oldest at a higher elevation,indicates lowering of base-level either related totectonic uplift of the hangingwall of the HFT or ahigh water:sediment ratio. Undated flights ofuplifted strath terraces which terminate alongthe HFT indicate the reactivation of this thrust atvarious time intervals (Nakata, 1972). Wesnouskyet al. (1999) calculated an uplift rate of

NNorthwestern Sector Southeastern Sector

Axial river

Axial river

NT

HFT

NT

HFT

D 20-16 ka

Axial river

Axial river

NT

HFT

NT

HFT

E 16-5 ka

Axial riverAxial river

NT

HFT

NT

HFT

F 4.5 ka- Present

Qf1

Qf2

Qf1Qf2

Qf1

Qf2

Qf1

Qf2

Qf1Qf2

Qf1Qf2

T1

T2T2

T1

T1

Second order fanT1 Terrace

First order fan

T2 TerraceRiver Channel

Qf1Qf2

NTHFT

Nalagarh ThrustHimalayan Frontal Thrust

Legend

Qf2 incision in both the sectors during last glacialmaxima arid climate and weak SW monsoon.

Reactivation of SW monsoon, erosion of regolith anddeposition on T1 terrace around 16 ka in entrenchedstreams in both southeastern and northwesternsectors. Initiation of interfan valley streams. Incisionbetween 14 and 5 ka .

Deposition on T2 terrace in both the northwestern andsoutheastern sectors. Incision continues, couplingbetween mountain streams and axial river and periodof non-deposition .

Fig. 12. Continued



69 mm year)1 for the HFT using an AMS 14C ageon charcoal (366 0215 ka) from an upliftedfluvial terrace at Mohand, SW of Dehra Dun(about 200 km east of Pinjaur Town). Tectonicallyinduced base-level change may be temporallyindependent of climatic change and alluviationrate and should show spatial variability along andbetween fault segments. The fan entrenchmentand subsequent terrace sedimentation in theintramontane valley shows no spatial variabilityeven though the valley is bounded by the seg-mented HFT.Climate change controls water and sediment

supply, and fan response is likely to be regionaland synchronous. The evidence for stream inci-sion and the absence of sedimentation in thevalley between 20 and 16 ka suggests low riverdischarge and reduced sediment supply or poss-ibly reduced sediment supply but relatively highstream power. The available data on the varia-tions of SW monsoon from the Arabian Sea andBay of Bengal (Cullen, 1981; van Campo et al.,1982; Duplessy, 1982; Sirocko et al., 1991) indi-cate that immediately following the LGM (2016 ka), the SW monsoon was weak resulting inless sediment discharge from Himalayan rivers.A subsequent increase in strength of the SW

monsoon after the late glacial maximum in-creased erosion and sediment supply and provi-ded sediment for deposition on the T1 terrace(Fig. 12E). However, the terrace deposits did notdevelop into new fan segments because of theproximity of the axial river to the toe of the Qf2fans, and the inter-glacial warm climate whichproduced more vegetation in the source area andless erosion than previous humid episodes. Har-vey (1984, 1990) reported that major periods offan aggradation coincided with Quaternary coldphases, and dissection with periods of lowersediment supply during warmer phases. The non-deposition between the T1 and T2 terraces indi-cate a prolonged incision phase between 15 and5 ka and is in agreement with reduced sedimentavailability on the mountain slopes due toincreased vegetation cover during the humidinter-glacial period. The last level of terrace (T2)deposition around 45 ka indicates increasedsediment availability probably due to a changefrom a humid to an arid climate (Fig. 12F). Theabsence of any new active fan lobes younger thanthe Qf2 suggests coupling between the mountaincatchments and downstream axial drainage undera more humid interglacial climate.Correlation with time equivalent aggradation

and incision phases on the Indo-Gangetic Plain,

south of the HFT, shows some parallel behaviour(Fig. 13). Prominent geomorphological surfaces inthe Ganga plain have been interpreted as aresponse to Late Quaternary climatic variations(Shukla et al., 2001; Gibling et al., 2005; Sinhaet al., 2005), locally with active tectonics indu-cing deep incision during the Late PleistoceneHolocene (Agarwal et al., 2002; Srivastava et al.,2003). The sedimentation history of the southernGangetic Plain reveals a major aggradation phasebetween 90 and 27 ka punctuated by minor non-depositional and degradation phases (Fig. 13;Gibling et al., 2005). Subsequently, degradationor localized accumulation occurred around theLGM with major incision occurring between 15and 5 ka driven by an increase in monsoonprecipitation. Goodbred (2003) suggested thatthe Ganges dispersal system shows little apparentattenuation of sedimentary signals between theGanga plain and downstream depocentres in theGanga delta and deep-sea Bay of Bengal and henceare dominantly controlled by the dispersal systemresponding to hydrological variations related tothe south-west monsoon. Minor differences in thedepositional history between the Pinjaur Dun andthe Indo-Gangetic Plain are probably due to thefact that the Late Quaternary alluvial fans in theSub-Himalaya are influenced by both tectonicsand climatic variations, whereas the Indo-Ganget-

Fig. 13. Correlation of various dated stratigraphicevents from the Pinjaur Dun and southern GangeticPlain (Gibling et al., 2005) with strength of SW mon-soon (from Prell & Kutzbach, 1987).



ic Plain was primarily influenced by climaticfluctuations.

CONCLUSIONS

The evolution of the Quaternary fans in thePinjaur intramontane valley records both tectonicand climatic fluctuations. Tectonism producedand maintained the relief and accommodationnecessary for the fans to form and dictated theduration of deposition. Within these tectoniccycles, climate changes controlled the water andsediment discharge to initiate fan aggradation andperiods of incision. Therefore, the stratigraphicevolution of the alluvial fans and terrace depositsof the Pinjaur Dun suggest that whilst tectonicprocesses exerted the dominant control on fanevolution prior to 72 ka; climatic fluctuationswith minor tectonic events may have been themost important factor since then.The sedimentology and chronology of Late

Quaternary alluvial fans and terrace deposits inthe Pinjaur Dun demonstrate:

1 Reactivation of the NT before 965 253 ka toproduce relief in the source area which resultedin the deposition of early high-gradient alluvialfans in the Pinjaur Dun.2 The resulting Qf1 fan deposits (between

965 253 and 837 163 ka) were dominatedby debris flow deposits, in contrast to a series ofsecondary Qf2 fans (between 724 134 and245 49 ka) that were initiated following aperiod of fan entrenchment which were domin-ated by stream flow deposits.3 The entrenchment of Qf1 and Qf2 around 74

and 20 ka respectively was driven by climatechanges.4 Toe cutting and recession of Qf2 around 45 ka

indicate a fanward shift of the axial river due totilting of the valley towards NE in response torenewed activity on the NT.5 Prolonged stream incision occurred following

the Late Glacial Maximum with minor deposi-tional phases as fluvial depositional terracesduring 163 21 and 45 ka respectively.

ACKNOWLEDGEMENTS

We are grateful to the Director, Wadia Institute ofHimalayan Geology, for providing the necessaryfacilities and encouragement. We are thankful toboth the reviewers (Drs A. M. Harvey and Ruth

Robinson) for valuable comments and sugges-tions. Dr N. S. Virdi is thanked for fruitfuldiscussion. We are thankful to the Departmentof Science and Technology (DST, Government ofIndia) for financial help under Pinjaur Dunproject (ESS/CA/A9/1996-97).

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Manuscript received 8 December 2004; revisionaccepted 9 January 2007



2007 suresh et al, sedimentology

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