Palaeoclimatic response to Quaternary gravels: A case study from Kumaun Himalaya, India

Quaternary International 213 (2010) 33–43

Contents lists available at ScienceDirect

Quaternary International journal homepage: www.elsevier.com/locate/quaint

Palaeoclimatic response to Quaternary gravels: A case study from Kumaun Himalaya, India U.K. Shukla a, *, D.S. Bora b, C.K. Singh a a b

Department of Geology, Banaras Hindu University, Varanasi-221005, UP, India Schlumberger Asia Services Limited, DCS, Mumbai, India

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 15 September 2009

Study of Quaternary intramontane Bhimtal palaeolake deposits and the Gaula piedmont fan deposits representing sediment source to sink scenario respectively; have revealed that gravelly sequences have directly responded to climatic turnovers creating contrasting lithofacies associations. Humid climatic conditions favoured thickly developed river generated clast-supported gravelly deposits, composed of clasts derived from a distant source. These deposits show intense lateral and vertical changes in lithofacies constitution. In contrast, arid phases are represented by extensive debris flows composed of clasts derived from adjacent mountain slopes. Gravelly deposits systematically interbedded in the stratigraphic records have potential for palaeoclimatic studies that is sometimes ignored. Ó 2009 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction There have been intense and sudden palaeoclimatic perturbations in terms of rainfall and aridity affecting sedimentation in continental and marine realms during the Holocene. The evidence is embedded in the Quaternary sedimentary record. Proxies including speleothems, stable isotopes and pollen analysis are presently used to unravel palaeoclimatic changes. There is need to compare and correlate continental palaeoclimatic records with their marine counterparts. On the continents, palaeolake profiles preserve the most complete palaeoclimatic record coded in condensed sediment thickness, and provide opportunity for high resolution studies at millennium scale. However, the lacustrine sequences are commonly made up of interbedded lake muds and coarser clastic sand and gravel brought by the channels meeting them. Intimately interbedded in the stratigraphy, and sometimes forming more than 70% of the sedimentary record, they are often left unattended during climate analysis. Gravelly sequences are also thickly developed at mountain fronts forming alluvial piedmont fans. The alluvial fans, characterized mainly by gravelly deposits with a meager representation of sand and mud, evolve in direct response to climate and tectonic changes at the source. Despite their prominence in the Quaternary records, the gravelly deposits are not commonly investigated by

* Corresponding author. E-mail address: [email protected] (U.K. Shukla). 1040-6182/$ – see front matter Ó 2009 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2009.09.001

palaeoclimatologists, and many workers consider them as the products of short lived catastrophic events of local significance. This paper concentrates on the gravelly sediments and advocates their relevance in palaeoclimatic studies. An attempt has been made to investigate the piedmont fan and lake profiles to decipher and interpret climatic turnovers near the Pleistocene–Holocene boundary. Emphasis is given in this paper to clast size and lithology, bed-geometry, resulting lithofacies and relative provenance location. The investigated sites are strategically positioned to characterize provenance deep in the mountain hinterland, and the sediment receptacle, the Piedmont Zone, located at the mountain foot (Fig. 1). Palaeolake deposits preserve the palaeoclimatic and tectonic history of the mountainous provenance, whereas the piedmont succession reveals the sequence of events at the sink site near the mountain foot. Both sites, although not adjacent, represent the same climate belt characterized by monsoonal subtropical conditions through Quaternary times, and therefore provide a regional perspective to palaeoclimatic variations through time. 2. Geological setting The study area is located in south-central Kumaun Himalaya within the Outer Lesser Himalayan domains in the north and Gangetic Basin forming a part of Foreland Basin System in the south (Valdiya, 1988; Shukla, 2009) (Fig. 1). From the north, the Proterozoic Lesser Himalayan succession, comprising non-fossiliferous calcareous to arenaceous rocks of shallow marine origin, is thrust over the Neogene Siwalik succession along the Main Boundary Thrust (MBT) (Valdiya, 1988, 1992). The Neogene Siwalik rocks

34

U.K. Shukla et al. / Quaternary International 213 (2010) 33–43

Fig. 1. Geological map showing various lithotectonic units and the major fault/thrust planes (modified after Raiverman et al., 1983), forming the Himalayan front in the study area around Nainital. Rocks of different ages are separated by major thrust planes: Himalayan Frontal Fault (HFF), Main Boundary Thrust (MBT) and Main Central Thrust (MCT). The Indus Tsangpo Suture Zone (ITSZ) marks the collision boundary between the Indian and Tibetan Plates.

comprising fine-grained sandstone and multi-coloured siltmudstones of fluvial origin (Bora and Shukla, 2005; Shukla et al., 2009), in turn are thrust over the Piedmont Zone along the emergent Himalayan Frontal Fault (HFF) forming a steep mountain front (Nakata, 1972; Valdiaya, 2003) (Fig. 1). The Bhimtal palaeolake basin is developed in an intramontane setting within the Lesser Himalaya, whereas the Piedmont Zone, forming the northernmost geomorphic element of the Gangetic Basin, juxtaposes the Siwalik mountain chain forming the Outer Himalaya, from the south (Kotlia et al., 1997; Shukla, 2009) (Figs. 1 and 2). The Piedmont Zone forms a 30–50 km wide narrow belt all along the Himalayan front from east to west. Many rivers emanating from the Lesser Himalaya housing the palaeolake valleys and the Siwalik Hills flow south through the Piedmont Zone, and deposit gravels filling narrow incised valleys (Shukla and Bora, 2003). Due to incision by these rivers, tens of kilometer-long cliffs exposing gravelly piedmont deposits are formed. 3. Palaeolake deposits An intramontane palaeolake succession developed in the Bhimtal area of Nainital district (29 200 N; 79 400 E) was investigated (Figs. 1 and 2A). The succession is exposed along a stream cutting across the palaeolake deposits near Bilaspur (Fig. 2B). The lake sediments are exposed at the height of 1432 m asl. The approximately 48 m thick succession is made up of repetitive units of grey muds and gravel deposits. Up to 27 m from the base, the sequence is gravel rich. Above this level, it is dominated by sand, silt and mud facies imparting an overall fining upward character to the palaeolake sequence. However, the recurring major facies in the lake succession are superimposed to form coarsening upward, 5–8 m thick cycles, where channelized or mud-supported gravels overlie mud deposits (Fig. 3). In the lateral association, the

facies exhibit changing characteristics from one end of the section to the other. Over the steeply dipping basement rocks on the SW margin, almost the entire thickness (about 38 m) of the sequence is made up of gravels (Fig. 4). The matrix-supported gravels resting directly over the basaltic basement at the valley mouth shows a depositional dip of 15 N. In the down-dip direction, towards N–NE at the 22.5 m level, the clast-supported gravels are overlapped by laterally persistent silty-clay horizons (Fig. 4). Kotlia et al. (1997) dated the mud horizons by 14C at ca. 25 ka in the middle to ca. 4 ka near the top of the succession (Figs. 3 and 4). For 14C dating, mud and silty clays containing abundant charcoal fragments were collected and dated in the Radiocarbon Laboratory of the Birbal Sahni Institute of Palaeobotany, Lucknow (India), following methods described by Rajagopalan et al. (1978) and Bradley et al. (1995) (also see Kotlia et al., 1997). A closer look at the gravelly units, however, reveals that they are not alike. They exhibit varied clast composition, texture, geometry and mode of emplacement. The gravelly horizons occurring at base of the profile up to 9 m (Figs. 3A and 5A), and at 36 and 45 m levels respectively are 4–9 m thick (Figs. 3E and 5C), multistoried and lobate to sheet-like in geometry. In multiple flows, individual stories are often separated by lensoidal sandy beds (Fig. 5A). They are matrix-supported and composed of clasts derived from the grey to green basalts forming the lake valley slopes within the intramontane setting (Figs. 2A and 4). Supported in silty–muddy matrix, 3–20 cm sized clasts are angular and unorganized without any facies differentiation, and large-scale slump structures are present. Gravels in the upper levels are red due to intense iron pigmentation and are devoid of organic matter. These gravelly horizons have sharp and occasionally deformed contacts with the associated mud units. The uppermost matrix-supported gravels filling the lake valley are bracketed at the base by a mud unit dated between 3 and 4 ka (Figs. 3F and 4).

U.K. Shukla et al. / Quaternary International 213 (2010) 33–43

35

Fig. 2. (A) Bhimtal palaeolake fill showing flat valley surface amid the surrounding hills. (B) Quaternary palaeolake deposits and existing lakes in the study area around Bhimtal (modified after Shukla and Bora, 2005).

In contrast, 5–12 m thick gravel units occurring at the 12 and 24 m levels (Figs. 3 and 4), respectively, are well organized and imbricated, containing clasts of volcanics, white to pink quartzites, grey silty shales and pink stromatolitic limestone derived from nearby mountain ridges exposing Proterozoic rocks (Figs. 1 and 2). Clasts are sub-angular to sub-rounded and range in size from 1 cm to 7 cm or more (Figs. 3B,C and 5B). These gravelly units are characterized by erosional lower contacts, lensoidal geometry, multistoried nature, fining upward unit cycles (FU), and large-scale 10–30 cm thick cross-bedding. The channel axes are aligned E–W and NNE–SSW, with palaeocurrents directed SSW to SW (Fig. 4). Decimeter to meter-thick gravelly sand lenses showing 3–7 cm current and wave affected ripples, parallel lamination and deformation features are incorporated at different levels within the gravel bodies (Figs. 3B and 5B). The intervening mud units at these levels are thickly developed (1.5–12 m thick), and contain gravel lenses and show interfingering with them (Figs. 3D and 4). Muds are parallel-laminated, some carbonaceous, weakly mottled, and contain rich paleontological assemblages comprising both arboreal and non-arboreal taxa. The upper part of the succession above 36 m level, corresponding to 18 m from the top of the sediment profile of Kotlia et al. (1997), is palynologically barren. Contacts between the mud and the overlying gravelly units are deeply erosional with extensive synsedimentary deformation. Laterally, these mud units commonly interfinger with gravelly deposits. One such mud unit occurring at 22.5 m is highly carbonaceous, contains charcoal, mottled texture

and differential iron pigmentation, and is dated between 25 and 26 ka (Figs. 3C and 4). Another thickly developed mud unit at 28 m showing coarsening upward character is dated between 21 and 22 ka (Figs. 3 and 4). At this level, the sequence becomes almost horizontal and the thickness of muddy horizons progressively increases towards south. The mud horizons are laterally persistent for several tens to hundreds of meters and pinch out in an E–W direction. The subsequent mud horizons separated by gravel sequences successively off lap the underlying ones (Fig. 4). 3.1. Interpretation The Bhimtal palaeolake succession is characterized by interbedded gravel and mud deposits. In general, gravels dominate the mud, forming more than 65% of the succession by thickness (Fig. 3). The sedimentation took place by fluvio-lacustrine processes and propagating fans. The lake sedimentation is represented by mud units, which decrease in thickness and becoming sandier at upper levels in the succession. These lake deposits show parallel lamination, coarsening upward sediment profiles (Shukla and Bora, 2005), and preservation of organic matter including pollen grains. Deposition seems to have taken place in deeper parts to sloping sandy–gravelly marginal parts of the lake. Preservation of organic matter within muds suggested that in deeper parts of the lake eutrophic conditions would have been prevailed, at least temporarily. Similar lake deposits have been described by Blair (1999) from Lake Lahontan Churchill Butte, Nevada, USA.

36

U.K. Shukla et al. / Quaternary International 213 (2010) 33–43

Fig. 3. Litholog of the Bhimtal palaeoke sequence exhibiting major lithofacies; and 14C dates after Kotlia et al. (1997). (A) Multiple flows of mud-supported gravels at the base of the palaeolake sequence. (B) Channelized units of parallel laminated–rippled sand containing dispersed gravels. (C) Multistoried clast-supported gravels eroding into pedogenic mud (Mollisol). (D) Coarsening upward lake mud deposit showing parallel lamination and lensoid gravelly lenses near the top. (E) Ferruginized matrix-supported gravels containing basalt fragments. (F) Sandy and mottled mud near the top of the palaeolake profile.

Gravelly deposits occurring at different levels in the lake profile show varied characters and mode of sedimentation. The matrixsupported gravels occurring at the base (0–9 m level) and near the top between 36 and 45 m levels respectively represent debris flows and are considered to be deposited during arid phases (Anderson and Sitar, 1995; Major, 1997; Shukla and Bora, 2005). The disintegrated clastic material present on the valley slopes that are made up of basaltic rocks was emplaced as high- and low viscosity sediment lobes approaching lake levels (Kim and Lowe, 2004; Bardou et al., 2007). The multistoried character, contrast in clast size and presence of intervening sandy units may imply their episodic emplacement. The interbedded lake mud units are thinner and sandy indicating reduced water budget and increased erosion due to lack of vegetation causing rapid siltation of lakes under a dry climate (cf. Srivastava et al., 2003) (Figs. 3 and 4). In contrast, the clast-supported and cross-bedded gravels having varied compositions were brought by the channels from

a distant source (Shukla and Bora, 2005). Therefore, the horizons occurring at 12–24 m levels (Figs. 3B,C and 5B) are interpreted to represent humid phases when enough water and current energy was available to transport the sediments as bedload in channels (Blair, 1999; Lunt and Bridge, 2004, 2007). Presence of imbrications, large-scale cross-bedding and effective separation of sandy facies from gravels requires energetic sustained flows in perennial channels (Krapf et al., 2005; Lunt and Bridge, 2007). However, steep slopes within mountainous terrain might have facilitated the transportation of gravels even during low discharge conditions under monsoonal climate (Pyrce and Ashmore, 2005; Lunt and Bridge, 2007). Nevertheless, the multistoried character with erosional mutual contacts of clast-supported units may imply lateral migration of channels within alluvial fan environments, interacting with the standing lake body (Shukla et al., 2002; Shukla and Bora, 2005; Shukla, 2009). The mudstone unit dated between 25 and 26 ka represents a soil horizon classed as a ‘Mollisol’

U.K. Shukla et al. / Quaternary International 213 (2010) 33–43

37

Fig. 4. Schematic lateral profile of the Bhimtal palaeolake fill showing mutual relationship of different lithounits and 14C dates. Mudflows at the base on the SW corner and resting over basaltic rocks show steep depositional dips directed to north. Lensoidal and sheet like gravelly units are overlapping laterally persistent lake muds. Palaeocurrents are directed towards the SSW and SW.

(Brady and Weil, 1996; Buol et al., 2003). This laterally persistent mud unit shows vertical zonation of lithology from carbonaceous mud to silty clay (Fig. 5F), differential ferruginisation, extensive charcoal, high degree of animal and plant burrowing representing weakly to moderately developed soil formed under waterlogged conditions preferably under humid to sub-humid climates

(Brady and Weil, 1996; Buol et al., 2003). Such soils are reported from mountain valleys covered with grasses, are still forming in flat based paleolacustrine valleys of the Himalaya, and can be seen as mottled and carbonaceous silt–mud cover cm–dm thick draped over the valley fill deposits. The humid climate is also corroborated by the increased thickness of lake mud horizons indicating the

Fig. 5. (A) Multiple debris flows separated by sandy layers (arrow) and also showing contrast in clast size (marked by dotted lines) at the base of the Bhimtal palaeolake sequence; scale, length of the encircled scraper is 25 cm. Angular clasts are suspended silt–clay matrix. (B) Clast-supported multistoried gravels and sandy interbeds at 15 m level in the sequence. (C) Ferruginized debris flow at the top of the palaeolake valley at 45 m level. A few cm sized angular clasts are embedded in clayey-silt matrix (D) Debris flow overlying the channelized gravels of Gaula piedmont fan. Different sized clasts are suspended in muddy matrix; also note the occurrence of sandy lenses marked by arrow; person for scale 156 cm. (E) Thinly bedded and laminated mud at 30 m level in Bhimtal palaeolake succession; scale, pencil length 14 cm. (F) Laterally persistent and carbonaceous pedogenic mud unit is erosionally (arrow) followed by channelized gravels.

38

U.K. Shukla et al. / Quaternary International 213 (2010) 33–43

existence of a larger lake around 21–22 ka (Figs. 3 and 4). The preserved thickness of mud horizons imply that Bhimtal Lake was at least 10–12 m deep, surrounded by sloping marginal parts where sand and gravels were deposited. Similar deductions relating lake levels with climate have been made by Benson et al. (1990) from Great Basin lake systems of North America. Northwardly directed depositional dip of the basal debris flows and southwardly directed palaeocurrents in clast-supported gravels indicate that the locus of sediment supply changed with time from south to north (Fig. 4). In the beginning, the mudsupported gravels were emplaced from the south as debris flows. Afterwards, channels flowing from the north, northeast and northwest transported sediments to the lake basin. The silt–mud sequences representing lacustrine conditions occur at five different levels partly or fully covering the thick clastic wedges of gravels (Fig. 4). Such a situation demands lake level fluctuation. The lake level fluctuation seems related to climatic turnovers that ultimately governed the water budget, sedimentation process and the lake dimension. Deposition of coarse clastics near the lake margin by propagating fans may have caused the lake level to rise further and deposit fine-grained material draping over the clastic wedges. The small-scale fluctuations, however, may also be related to seasonality under a monsoonal climate producing smaller sedimentation cycles within the lake. The existing lakes of the study area show meter scale water level fluctuations during monsoon months. 4. Piedmont fan deposits The Piedmont Zone is the northernmost geomorphic element in the Indo-Gangetic Basin (Shukla and Bora, 2003; Shukla, 2009). It is developed at the Himalayan foothills and shows a thrusted contact along HFF with the Siwalik rocks forming the mountain front (Valdiaya, 2003) (Fig. 1). The Piedmont Zone is the sink for the coarse clastics supplied from the Himalayan source. It is made up of numerous coalescing alluvial fans recording the signatures of hinterland tectonism and palaeoclimate (Fig. 6). For the purpose of the present study, an extensive survey of the Piedmont Zone was made and the Gaula piedmont fan studied. The Gaula piedmont fan is developed at the Himalayan foothills in Kathgodam–Haldwani–Lalkuan areas (Figs. 6 and 7). The Gaula Fan is located in a monsoonal subtropical region between 29 0 and 150 N, 79 0 and 200 E. Between 526 m and 246 m asl and covering 404 km2, the Gaula piedmont fan is 30 km long and gravel dominated. It shows intense facies variation from gravelly proximal (Bhabar region) to sand-clayey distal parts of the fan (Terai region) with progressive decrease in clast size and concurrent increase in the representation of sand, silt–mud in the down slope direction (Figs. 6 and 7). The Gaula piedmont fan is divisible into two lithological successions, clast-supported gravels and matrix-supported gravels (Figs. 8 and 9). The clast-supported gravels are 8–10 m thick, cross-bedded and channelized. They grade into gravel-sand to silt–mud from proximal to mid and distal fan areas respectively (Figs. 9 and 10). They are composed of clast derived from quartzites, stromatolitic limestones, metamorphics, basalt and Siwalik rocks forming the mountain hinterland. A systematic decrease in clast size occurs from an average of 10–20 cm in the proximal part to granules in the midfan. The distal parts of the fan are devoid of gravels and characterized by interbeds of sand, silt and mud (Fig. 10). Lateral profiling of a 16 m high cliff section in the proximal part of the fan provided opportunity to characterize the facies and genetic surfaces of different orders described by Miall (1996) and others (Fig. 9). The clast-supported gravel succession shows a number of small- to large-scale erosional surfaces (Fig. 8A). These erosional surfaces are marked by changes in clast size, facies,

Fig. 6. Schematic geomorphic map at 1:50 000 scale of the Piedmont Zone showing Gaula Fan and adjacent coalescing smaller piedmont fans. The Piedmont Zone is 10–40 km wide. Converging streams mark the down valley piedmont fan boundary (marked by A–A0 ).

truncation of units and change in the nature and dimension of primary physical structures. The higher order (4th to 5th order) erosion surfaces, extending for tens to hundreds of meters, are generally 80–150 m wide and have deep erosional bases representing episodes of channel migration across the fan (Fig. 9). The 4th order sequences are made of 3rd order surfaces that are 1.5–3 m thick, large-scale (8–40 cm thick) cross-bedded and represent unit bar (channel-bar) sequences (Miall, 1996). Inclined in different directions, the amalgamating unit bars are 20–40 m long and lenticular in geometry with erosional or nonerosional lower contacts. Constituted of planar cross-bedded, trough crossbedded and horizontally lithofacies, the unit bars show variable palaeocurrents directed southward with higher dispersion (Fig. 9).

Fig. 7. Gravelly Gaula piedmont fan succession facing the Siwalik mountain front along the Himalayan Frontal Fault (HFF) near Kathgodam–Haldwani areas. Due to incision of the Gaula River several kilometer long gravelly deposits are exposed forming 10–15 high cliff walls. Inset shows the close up of gravelly deposits containing horizontal gravel sheets and cross-beds.

U.K. Shukla et al. / Quaternary International 213 (2010) 33–43

39

Fig. 8. Sedimentological logs showing sedimentary structures and facies of Gaula Fan in the proximal part near Haldwani, (A) Channelized (marked by dashed lines) and clast-supported gravels forming multistoried complexes. (B) Matrix-supported gravels developed in the proximal part of the Gaula Fan overlying the clast-supported succession.

The overlying matrix-supported gravels are 2–5 m thick and follow the clast-supported gravels with a marked erosional contact of regional significance (Figs. 8B and 9), which may correspond to a 5th order surface of Miall (1996), and is equivalent to a sequence boundary of regional significance described by Embry (1990) and Miall and Arush (2001). Across this surface, the Gaula Fan shows differences in lithology, clast size and sediment texture and composition (Figs. 8 and 9). Therefore, such surfaces may have tectonic and/or palaeoclimatic significance. The matrix-supported gravels of proximal and mid fan areas grade sharply to a mud dominant succession in the distal fan area (Fig. 10). The matrixsupported gravels are exclusively constituted of Lower Siwalik sandstone and shales juxtaposing the Piedmont Zone along a thrusted contact (Figs. 5D and 7). At places, three to four gravelly units (3–5 m thick) are separated by ferruginized silt–sand lenses lacking any perceptible pedogenic modifications. Sub-angular to angular clasts of varied dimension including meter sized fragments are

suspended in muddy matrix or clustered due to slumping (Figs. 5D and 8B). The fan surface is draped by a meter thick carbonaceous silt–mud apron increasing in thickness from proximal to distal fan. 4.1. Interpretation The Gaula Fan succession seems to have developed under changing condition of climate and tectonics (Shukla and Bora, 2003). The lithological succession characterized by clast-supported gravels and showing a multistoried channelized pattern is interpreted to have deposited under alluvial fan environment by fluvial processes (Shukla, 2009) (Fig. 10). Laterally migrating channels mainly in response to increased sediment supply produced multistoried gravel bodies (Blair and McPherson, 1994; Krapf et al., 2005) (Figs. 8 and 9). Multistoried cross-cutting character of gravelly units, amalgamating unit bar pattern and higher palaeocurrent dispersion suggest that channels were mostly braided (Shukla et al., 1999;

40

U.K. Shukla et al. / Quaternary International 213 (2010) 33–43

Fig. 9. Schematic lateral profile of proximal part of the Gaula Fan showing lithofacies distribution and surfaces of genetic significance (1st to 5th order surfaces) grouped in to six accretionary units. Separated by a 5th order surface of regional significance, channelized gravelly succession is overlain by debris flow deposits.

Lunt and Bridge, 2004, 2007). The thickness of the sedimentary packages (Fig. 8) and the lateral extent of 4th order surfaces (Fig. 9) suggest that the individual channels in the active channel belt were 2–3 m deep and about 100 m wide. The water volume required for transporting coarse clastics from a relatively distant sediment source located deep into the Lesser Himalayan domain as bedload, and to generate large-scale bedforms, was consistently available (Lunt and Bridge, 2004, 2007; Shukla, 2009). The thickness of crossbeds and clast size involved require a consistent flow velocity of more than 60–100 cm/s (Nichols, 2009). Effective separation of gravelly, sandy and silt-clayey dominated sequences from the proximal through middle to distal parts of the fan within a distance of about 30 km (Fig. 10) indicates progressive sorting and consistent flow conditions for a considerable period (Brozovic and Burbank, 2000). As compared to clast-supported gravels, the matrix-supported gravelly succession indicates emplacement by gravity flow processes (Blair and McPherson, 1994; Major, 1997; Kim and Lowe, 2004). Multiple mud-supported-gravels composed exclusively of

Fig. 10. Lateral facies changes in river generated succession of the Gaula fan during the humid phases. Note the gradation from gravel to gravel-sand and silt–mud in the progressively down flow direction along the fan radii (A). During debris flows in the proximal parts of the fan, thick silt–mud sedimentation takes place in the distal parts of the fan (B). In the mountain hinterland a major thrust plane the Himalayan Frontal Fault (HFF) is shown.

Siwalik sandstones were deposited as mudflows under reduced water budget conditions (Brozovic and Burbank, 2000; Bardou et al., 2007; Shukla, 2009) (Figs. 5D and 8B). The Siwalik succession juxtaposing the Piedmont Zone along a faulted contact, and made up of sandstone–mudstone couplets (Bora and Shukla, 2005; Shukla et al., 2009), might have provided ample mud content to facilitate mudflows during occasional catastrophic rains under arid to sub-arid climate (Figs. 7 and 10). The individual mudflow events are commonly separated by lensoid silt–mud beds indicating periods of surface runoff (Brozovic and Burbank, 2000; Bardou et al., 2007). The mud-supported gravel deposits in the proximal part of the fan abruptly grade into mud-dominated deposits in the distal parts of the fan, which may be related to later reworking of the debris flows (Fig. 10). The uppermost silt–mud cover which seems unrelated to fan building activity represents sheet flows, operative during rains on the sloping fan surfaces during ensuing ameliorated climate (Shukla, 2009). 5. Discussion Gravelly deposits are very common in inter-to-intramontane basins and piedmont setting near the mountain foot. Because of nearness of the sediment source and high gradients, the gravels ranging in size from granules to boulders are easily emplaced by various processes induced due to changing set of climate and tectonic conditions. Palaeolake basins represented by exceptionally flat areas amid rugged mountain terrains had been potential sites for gravel accumulation within the Himalaya through Quaternary times. The lake basins in such a setting are often created by damming river courses and valley mouths by mass flow events triggered by cloud bursts, and/or block movements along active faults traversing the area. Lakes may also receive coarse clastics from channels. Therefore, the mountainous palaeolake deposits located within the lesser Himalayan domain (Fig. 2), and characterized by association of gravelly and silt–mud sequences (Fig. 3), offer special opportunities to study closely the interaction between climate, tectonics and sedimentation. Similarly, the Piedmont Zone making the northernmost geomorphic element of the Himalayan foreland basin (Fig. 6) and receiving its sediment budget from the Lesser and Outer Himalayan hinterlands is another potential domain to diagnose and relate the climatic–tectonic turnovers between source, the Himalayan hinterland (Figs. 1 and 2), and sink, the Piedmont Zone (Figs. 6 and 7). Unfortunately, however, due to meager representation of fine-grained silt and clay preserved

U.K. Shukla et al. / Quaternary International 213 (2010) 33–43

datable organic matter, comprehensive dating of the gravelly piedmont deposits is yet not available. Furthermore, large-scale cannibalization of amalgamated lithotectonic units resulting in homogenization of mineralogical signatures during the Cenozoic has hampered the use of major and trace elements for provenance interpretation (Tripathia et al., 2007 and references therein). Therefore, clay mineralogy appears as a weak link to comment on the exact sediment sourcing for the Quaternary Himalayan foreland deposits. Nevertheless, the similarity in clast composition of the piedmont deposits with Bhimtal palaeolake sediments suggests that the sediment source for both the basins has been the Precambrian Lesser Himalaya in addition to Tertiary Siwalik belt for the former. The bulk of the sediment in both the studied sites are made of Lesser Himalayan quartzites, dolomites, limestones, silty shales and green basalts in addition to Lower Siwalik sandstones and shales in the Gaula piedmont deposits. At present, most of the piedmont rivers originate in the Lesser Himalayan domain, and pass through the Siwalik Hills before entering into the Gaula Piedmont Zone of the foreland basin to supply sediments. The stream cutting through the Bhimtal valley exposing palaeolake succession (Fig. 2) is an important tributary of the Gaula River incised to the Piedmont Zone (Figs. 6 and 7). The river could have supplied sediment for the formation of the Gaula piedmont fan during the Quaternary. Gaula piedmont fan and Bhimtal palaeolake represent a continuum of processes between two studied sites under prevailing climate–tectonic conditions though Quaternary times. Bhimtal palaeolake originated at about 40 ka due to blockage of a river by a huge debris flow, probably related to an important contemporary tectonic event in the area (Valdiya, 1992; Kotlia et al., 1997; Shukla et al., 2002). The debris flow that blocked the Bhimtal palaeolake is represented by 9 m thick mud-supported gravels composed almost exclusively of basalts at the base of the succession (Figs. 3A, 4 and 5A). These multiple mudflows indicate reduced water budget conditions representing an arid–semiarid climate coupled with intensified tectonics (Nemec and Postma, 1993; Brozovic and Burbank, 2000; Shukla et al., 2002; Kim and Lowe, 2004; Bardou et al., 2007). Establishment of a small lake initially is marked by a 3 m thick silt–mud unit succeeding the debris flows (Figs. 3 and 4). From 12 to about 24 m above the base, clast-supported gravels, laminated silt and rippled sand dominate. These gravelly units composed of mainly quartzite and limestone clasts exposed at a few kilometers away from the basin of deposition represent humid climate with enough water budgets and current energy to transport the clasts for a considerable distance (Carling, 1996; Shukla and Bora, 2003; Lunt and Bridge, 2004, 2007). Characterized by copious plant and animal activity, preservation of charcoal and differential mottling, the Mollisol horizon present at the 22.5 m level and associated thicker lake mud units also support a humid climate (Brady and Weil, 1996; Buol et al., 2003) (Figs. 3C,D and 4). Based on its basin wide development, the palaeosol is

41

believed to have developed when the lake was temporarily drained due to renewed tectonic activity in the basin area (Shukla and Bora, 2005). The lake sediments were probably exposed for a considerable time to generate the Mollisol unit under the influence of humid climate and vegetation. The debris flows near the top of the palaeolake succession with subordinate lake mud dated between 3 and 4 ka represent a renewed phase of aridity (Figs. 3 and 4). Thus, in the succession, two arid–semiarid phases are separated by a prolonged humid phase. The Gaula Fan sequence located several tens of kilometers south at the Himalayan front represents a comparable sequence of events (Figs. 8 and 9). The clast-supported gravelly sequence at the base represents deposition by rivers, indicating sedimentation in humid climate when the rivers had enough water and the hydraulic energy supported by steeper regional gradient to carry gravels (sized, 2–3 cm) for 20–22 km (cf. Lunt and Bridge, 2004, 2007; Shukla, 2009) (Fig. 7). In case of the Gaula Fan, sustained deeper flows in channels favoured development of high relief bedforms, producing large-scale cross-beds, horizontally bedded gravel sheets, and effective sorting of material to generate lateral facies differentiation (Blair and McPherson, 1994; Krapf et al., 2005; Shukla, 2009). In contrast, the mud-supported gravel succession implies relatively dry climate. Occasional catastrophic rains that are common in arid climate might have triggered the large-scale debris flows in combination with renewed tectonic activity along the Himalayan Frontal fault (HFF) (Blair and McPherson, 1994; Shukla, 2009). Thick mudstones present within the Siwalik succession forming the mountain front along and juxtaposing the Piedmont Zone might have facilitated debris flows containing muddy matrices (cf. Brozovic and Burbank, 2000; Bardou et al., 2007) (Figs. 7 and 10). Shukla (2009) identified such at least three debris flow events in the subsurface borehole profiles of Gaula Fan and ascribed them to represent fan expansion cycles formed during dry climate coupled with active tectonics at the Himalayan mountain front. Similar thick matrix-supported gravel deposits representing debris flows have been recorded along the Himalayan Foothills in Nainital (Kotabagh section), Pithoragarh (Kaloniya Nullah section) located east of Nainital and Bijnor district (Kotwali river section near Sabalgarh dated between 8 and 3 ka by Srivastava et al., 2000), covering the older Quaternary sediments (Fig. 11). Between 35 and 20 ka, a strong monsoon in India and south Asia prevailed (Kutzbach, 1987). The majority of the Bhimtal palaeolake succession may belong to this humid phase. A prominent phase of aridity started at about 18 ka (Van Campo, 1986) and lasted up to about 12 ka (Srivastava and Shukla, 2009). Punctuated by a brief humid period during the Early Holocene, a phase of decreased monsoons and increased aridity during 6–3 ka is marked by extensive calcretization of upland terrace surface sediments, by abandonment of active channel belts and formation of extensive ponds and lakes. Deposition of aeolian sand ridges has been

Fig. 11. Kotwali river section (Bijnor district in Fig. 1) showing debris flows dated between 8–3 ka by Srivastava et al. (2003).

42

U.K. Shukla et al. / Quaternary International 213 (2010) 33–43

recorded from the Gangetic foreland basin which acted as a receptacle for sediments shed from the Himalayas (Singh et al., 1999; Srivastava et al., 2000, 2003). Therefore, clast-supported gravels representing a humid climate within the piedmont succession may correspond to climatic amelioration around 13–6 ka (Sirocko et al., 1991; Srivastava et al., 2003; Shukla and Bora, 2005). The water budget of the rivers was increased due to enhanced melting of Himalayan glaciers and the re-establishment of the SW monsoon. The overlying mudflow event in the Gaula Fan and the upper debris flows in Bhimtal lake may correspond to the 6–3 ka arid phase. The period between 7 and 4 ka is also marked by an important tectonic event causing disruption of palaeolake basins and their subsequent filling by mudflows in Kumaun and Garhwal Himalaya (Shukla et al., 2002; Singh, 2004; Shukla and Bora, 2005). Bhimtal palaeolake seems to have drained during 3–4 ka due to movement along a WNW–ESE trending fault traversing the basin area (Kotlia et al., 1997; Shukla and Bora, 2005). In the Gaula piedmont succession, this tectonic event is indicated by the debris flows succession composed of Siwalik rocks facing the Piedmont Zone. Most likely, tectonic activity along Himalayan Frontal Fault (Nakata, 1972; Kumar et al., 2001) caused the upheaval of Siwalik rocks and released a huge amount of coarse clastics, subsequently emplaced as widespread debris flows under a dry climate. Both the Lesser Himalaya and Siwalik terrains have been tectonically active through late Pleistocene to Holocene times. Tectonic reactivation of the Main Boundary Thrust (MBT) and Himalayan Frontal Fault (HFF) has been dated in different areas by TL and radiocarbon methods. Sediments of the active fault zone of the MBT near Nainital have a reactivation age of ca. 70 ka BP, whereas, in Tons valley and Himachal foothills the ages are clustered between ca. 20 and 40 ka BP (Valdiya, 1992). Palaeoliquefaction studies in northern Bihar have shown the recurrence of seismicity along the HFF around 25, 000 yr BP, and later between ca. 1700 and 5300 yr BP respectively (Valdiaya, 2003). In the Yamuna valley, significant neotectonic activity is dated at ca. 4 ka BP along the HFF (Valdiya, 2001). Therefore, it seems that along with the climate, tectonics played an important role in sediment production and sourcing to the Piedmont Zone of the Himalayan Foreland Basin. Acknowledgments Thanks are expressed to the Head of the Department of Geology, Banaras Hindu University, Varanasi, for providing the working facilities of the Department, and SAP funds. Initially, the study was financed by D.S.T., New Delhi through a Young Scientist Project (HR/OY/A-12/95) awarded to UKS. References Anderson, S.A., Sitar, N., 1995. Analysis of rainfall-induced debris flows. Journal of Geotechnical Engineering – ASCE 121, 544–552. Bardou, E., Boivin, P., Pfeifer, H.R., 2007. Properties of debris flow deposits and source materials compared: implications for debris flow characterization. Sedimentology 54, 469–480. Benson, L.V., Currey, D.R., Dorn, R.I., Lajoie, K.R., Oviatt, C.G., Robinson, S.W., Smith, G.I., Stine, S., 1990. Chronology of expansion and contraction of four Great Basin lake systems during the past 35,000 years. Palaeogeography, Palaeoclimatology, Palaeoecology 78, 241–286. Blair, T.C., 1999. Sedimentology of gravelly Lake Lahontan highstand shoreline deposits, Churchill Butte, Nevada, USA. Sedimentary Geology 123, 199–218. Blair, T.C., McPherson, J.G., 1994. Alluvial fans and their natural distinction from rivers based on morphology, hydraulic processes, sedimentary processes and facies assemblages. Journal of Sedimentary Research 3, 433–489. Bora, D.S., Shukla, U.K., 2005. Petrofacies Implication for the Lowwer Siwalik Foreland Basin Evolution, Kumaun Himalaya, India. In: Palaeontological Society of India Special Publication, 2 163–179. Brady, N.C., Weil, R.R., 1996. The Nature and Properties of Soils, 11th ed. Prentice Hall, New Jersey, 960 pp.

Bradley, R.S., Dodson, J., Duplessy, J.C., Gasse, F., Liu, T.S., Markgra, V., 1995. PANASHPEP science and implementation. Palaeoclimates of the Northern and Southern Hemispheres. PAGES 95, 91–105. Brozovic, N., Burbank, D.W., 2000. Dynamic fluvial systems and gravel progradation in the Himalayan foreland. Geological Society of America Bulletin 112, 394–412. Buol, S.W., Southard, R.J., Graham, R.C., McDaniel, P.A., 2003. Soil Genesis and Classification, fifth ed. Iowa State University Press – Blackwell Publ., 494 pp Carling, P.A., 1996. Morphology, sedimentology and palaeohydraulic significance of large gravel dunes, Altai Mountains, Siberia. Sedimentology 43, 647–664. Embry, A.F., 1990. A tectonic origin for third-order depositional sequences I extensional basins––implications for basin modeling. In: Cross, T.A. (Ed.), Quantitative Dynamic Stratigraphic. Prentice Hall, Englewood Cliffs, pp. 491–501. Kim, B.C., Lowe, D.R., 2004. Depositional processes of the gravelly debris flow deposits, South Dolomite alluvial fan, Owens Valley, California. Geosciences Journal 8, 153–170. Kotlia, B.S., Bhalla, M.S., Sharma, C., Rajagopalan, G., Ramesh, R., Chauhan, M.S., Mathur, P.D., Bhandari, S., Chacko, S.T., 1997. Palaeoclimatic conditions in the Upper Pleistocene–Holocene Bhimtal–Naukuchiatal lake basins in southcentral Kumaun, north India. Palaeogeography, Palaeoclimatology, Palaeoecology 130, 307–322. Krapf, C.B.E., Stanistreet, I.G., Stollhofen, H., 2005. Morphology and Fluvio-Aeolian Interaction of the Tropical Latitude, Ephemeral Braided-River Dominated Koigab Fan, North-West Namibia. In: International Association of Sedimentologists Special Publication, 35 99–120. Kumar, S., Wesnousky, S.G., Rockwell, T.K., Ragona, D., Thakur, V.C., Seitz, G.G., 2001. Earthquake recurrence and rupture dynamics of Himalayan Frontal Thrust, India. Science 249, 2328–2331. Kutzbach, J.E., 1987. The changing pulse of the monsoon. In: Phein, J.S., Stephens, P.L. (Eds.), Monsoons. Wiley, New York, pp. 247–268. Lunt, I.A., Bridge, J.S., 2004. Evolution and deposits of a gravelly braid bar, Sagavanirktok River, Alaska. Sedimentology 51, 415–432. Lunt, I.A., Bridge, J.S., 2007. Formation and preservation of open-framework gravel strata in unidirectional flow. Sedimentology 54, 71–87. Major, J.J., 1997. Depositional processes in large-scale debris flow experiments. Journal of Geology 105, 345–366. Miall, A.D., 1996. The Geology of Fluvial Deposits. Springer, Berlin, 582 pp. Miall, A.D., Arush, M., 2001. Cryptic sequence boundaries in braided fluvial successions. Sedimentology 48, 971–985. Nakata, T., 1972. Geomorphic History and Crustal Movements of the Foot Hills of Himalaya. Institute of Geography, Tohuku University of Sendai, pp. 162–164. Nemec, W., Postma, G., 1993. Quaternary alluvial fans in southern Crete: sedimentation processes and geomorphic evolution. In: Maego, M., Puigdeforbregas, C. (Eds.), Alluvial Sedimentation. International Association Sedimentologists, Special Publication, 17, pp. 235–276. Nichols, G., 2009. Sedimentology and Stratigraphy, second ed. Wiley–Blackwell, 419 pp. Pyrce, R.S., Ashmore, P.E., 2005. Bedload path length and point bar development in gravel-bed river models. Sedimentology 52, 839–857. Raiverman, V., Kunte, S.V., Mukherjea, A., 1983. Basin geometry, Cenozoic sedimentation and hydrocarbon prospects in northwestern Himalaya and IndoGangetic Plains. Petroleum Asia Journal 6, 67–92. Rajagopalan, G., Vishnu-Mittre, Sekar, B., 1978. Birbal Sahni Institute radiocarbon measurements. Radiocarbon 20, 398–404. Shukla, U.K., 2009. Sedimentation model of gravel-dominated alluvial piedmont fan, Ganga Plain, India. International Journal of Earth Sciences (Geol Rundsch) 98, 443–459. Shukla, U.K., Bora, D.S., 2003. Geomorphology and sedimentology of piedmont zone, Ganga Plain, India. Current Science 84, 1034–1040. Shukla, U.K., Bora, D.S., 2005. Sedimentation model for the Quaternary intramontane Bhimtal–Naukuchiatal lake deposits, Nainital, India. Journal of Asian Earth Sciences 25, 837–848. Shukla, U.K., Bora, D.S., Singh, C.K., 2009. Geomorphic positioning and depositional dynamics of river systems in Lower Siwalik basin, Kumaun Himalaya. Journal of the Geological Society of India 73, 335–354. Shukla, U.K., Kotlia, B.S., Mathur, P.D., 2002. Sedimentation pattern in a Trans Himalayan Quaternary lake at Lamayuru (Ladakh), India. Sedimentary Geology 148, 405–425. Shukla, U.K., Singh, I.B., Srivastava, P., Singh, D.S., 1999. Palaeocurrent patterns in braid and point bar deposits: examples from Ganga River, India. Journal of Sedimentary Research 69, 992–1002. Singh, I.B., 2004. Late Quaternary history of the Ganga Plain. Journal of the Geological Society of India 64, 431–454. Singh, I.B., Srivastava, P., Sharma, S., Sharma, M., Singh, D.S., Rajagopalan, G., Shukla, U.K., 1999. Upland interfluve (Doab) deposition: alternative model to muddy overbank deposits. Facies 40, 197–210. Sirocko, F., Sarnthein, M., Lange, H., Erlenkeuser, H., 1991. Atmospheric summer circulation and coastal up welling in the Arabian Sea during the Holocene and last glaciation. Quaternary Research 36, 72–93. Srivastava, P., Shukla, U.K., 2009. Quaternary evolution of the Ganga River system: new quartz ages and a review of Luminescence Chronology. Himalayan Geology 3, 85–94. Srivastava, P., Shukla, U.K., Mishra, P., Sharma, M., Sharma, S., Singh, I.B., Singhvi, A.K., 2000. Luminescence chronology and facies development of Bhur sands in the interfluve region of Central Ganga Plain, India. Current Science 78, 498–503.

U.K. Shukla et al. / Quaternary International 213 (2010) 33–43 Srivastava, P., Singh, I.B., Sharma, S., Shukla, U.K., Singhvi, A.K., 2003. Late Pleistocene–Holocene hydrologic changes in the interfluve areas of the central Ganga Plain, India. Geomorphology 54, 279–292. Tripathia, J.K., Ghazanfarib, P., Rajamania, V., Tandon, S.K., 2007. Geochemistry of sediments of the Ganges alluvial plains: evidence of large-scale sediment recycling. Quaternary International 159, 119–130. Valdiya, K.S., 1988. Geology and Natural Environment of Nainital Hill, Kumaun Himalaya. Gyanodaya Prakashan, Nainital, 155 pp.

43

Valdiya, K.S., 1992. The main boundary thrust zone of the Himalaya, India. Annals of Tectonics (Special issue 6), 54–84. Valdiya, K.S., 2001. Reactivation of terrane-defining boundary thrusts in central sector of Himalaya: implications. Current Science 81, 101–114. Valdiaya, K.S., 2003. Reactivation of Himalayan Frontal fault: implications. Current Science 85 (7), 1031–1040. Van Campo, E., 1986. Monsoon fluctuation in two 20 000 yr. B.P. oxygen isotopepollen records of southwest India. Quaternary Research 26, 376–388.