Depositional environment and provenance of Middle

Siwalik sediments in Tista valley, Darjiling District,

Eastern Himalaya, India

Abhik Kundu1,∗ , Abdul Matin2 and Malay Mukul3

1
Department of Geology, Asutosh College, 92, S.P. Mukherjee Road, Kolkata 700 026, India.
2
Department of Geology, University of Calcutta, 35, Ballygunge Circular Road, Kolkata 700 029, India.
3
Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India.
∗
Corresponding author. e-mail: kundu.abhik@gmail.com

The frontal part of the active, wedge-shaped Indo-Eurasian collision boundary is defined by the
Himalayan fold-and-thrust belt whose foreland basin accumulated sediments that eventually became part
of the thrust belt and is presently exposed as the sedimentary rocks of the Siwalik Group. The rocks
of the Siwalik Group have been extensively studied in the western and Nepal Himalaya and have been
divided into the Lower, Middle and Upper Subgroups. In the Darjiling–Sikkim Himalaya, the Upper
Siwalik sequence is not exposed and the Middle Siwalik Subgroup exposed in the Tista river valley
of Darjiling Himalaya preserves a ∼325 m thick sequence of sandstone, conglomerate and shale. The
Middle Siwalik section has been repeated by a number of north dipping thrusts. The sedimentary facies
and facies associations within the lithostratigraphic column of the Middle Siwalik rocks show temporal
repetition of sedimentary facies associations suggesting oscillation between proximal-, mid- and distal
fan setups within a palaeo-alluvial fan depositional environment similar to the depositional setup of the
Siwalik sediments in other parts of the Himalaya. These oscillations are probably due to a combination
of foreland-ward movement of Himalayan thrusts, climatic variations and mountain-ward shift of fan-
apex due to erosion. The Middle Siwalik sediments were derived from Higher- and Lesser Himalayan
rocks. Mineral characteristics and modal analysis suggest that sedimentation occurred in humid climatic
conditions similar to the moist humid climate of the present day Eastern Himalaya.

1. Introduction preserved almost continuously since the Miocene

(Brozovic and Burbank 2000 and references
The collision of the Indian and Tibetan plates, therein). The Early Miocene–Pliocene sedimentary
initiated ∼55 Ma ago, resulted in the formation rocks are known as the Siwalik Group in India and
of the Himalayan orogen. The Himalayan foreland Pakistan and Churia Group in Nepal (Medlicott
basin has evolved since the Palaeogene time in 1864; Nakayama and Ulak 1999). The Siwalik
the frontal part of the orogen as the thrust belt Group of rocks is seen exposed in primarily
propagated towards its foreland and is currently two kinds of frontal settings. In the western
located in the Indo–Gangetic plains. The sediments Himalaya, the Siwalik rocks are exposed in a
deposited in the Himalayan foreland basin are ‘Dun-type’ frontal setting whose morphology is

Keywords. Eastern Himalaya; Middle Siwalik; foreland basin; facies; alluvial fan.

J. Earth Syst. Sci. 121, No. 1, February 2012, pp. 73–89

c Indian Academy of Sciences 73
74 Abhik Kundu et al

dominated by intermontane, longitudinal valleys Himalaya. Although the depositional setup of the
termed as the Duns (e.g., Pinjore Dun, Dehra Middle Siwalik Subgroup of the Tista valley has
Dun (Nakata 1972)), the easternmost of which is been worked out (Banerji and Banerji 1982),
the Trijuga Dun in Nepal (e.g., Kimura 1999). a detailed account of the lithology, sedimentary
The Duns are typically bounded by an east-west facies and the stratigraphy of the Middle Siwalik
trending mountain range associated with the Main Subgroup is not available. This study works out
Frontal Thrust (MFT). In contrast, the eastern the detailed sandstone petrography of the Middle
Himalaya is dominated by ‘imbricate-type’ fronts Siwalik sedimentary rocks exposed in the Tista
defined by an imbricate or schuppen zone of thrust valley for the first time. Modal analysis of frame-
faults that may or may not involve a unique MFT work minerals of sandstones for the first time
similar to the Dun-type front. The mountain front and data generated from facies analysis (Kundu
in this setting is defined by faults other than the et al 2011) to unveil the provenance characteristics,
MFT (e.g., the Ramgarh Thrust, Matin and Mukul depositional environment and palaeoclimate dur-
2010) or an imbricate or schuppen zone of thrust ing Middle Siwalik sedimentation in the Darjiling
faults that repeat the Siwalik section (e.g., Mukul Himalaya.
2000).
The Siwalik Group of rocks is exposed along
the Tista river and road sections in Tista valley 2. Geological setting
in the Darjiling Himalaya of West Bengal. Pilgrim
(1906); Heim and Gansser (1939); Ganguly and The major structural units of the Himalayan oro-
Rao (1970); Acharyya (1976, 1994); Banerji and gen from north to south are the South Tibet
Banerji (1982) have studied the Siwalik rocks of Detachment (STD), Main Central Thrust (MCT),
this area. The Siwalik sequence of the Tista valley the Ramgarh Thrust (RT), the Main Boundary
section (TVS) is subdivided into lower and upper Thrust (MBT) and the Main Frontal Thrust
units (Banerji and Banerji 1982). Siwalik sediments (MFT) (figure 1a and b; Yin 2006; Mukul 2010).
in this region have been interpreted as alluvial The MCT marks the boundary between Greater-
fan deposits (Acharyya 1976; Banerji and Banerji and Lesser Himalayan sequences and MBT marks
1982). the boundaries between the Lesser- and Sub-
Sedimentary facies is defined as sediments or Himalayan sequences (Gansser 1964). The MBT
sedimentary rocks that can be recognized and carries the Gondwana and Daling Group of rocks
distinguished from others by their geometry, over the Tertiary Siwalik sedimentary sequence in
lithological composition, sedimentary structures, its footwall (Heim and Gansser 1939; Yin 2006
palaeoflow patterns and fossils (Selley 1985). and references therein). The southernmost and
According to Reading and Levell (1996), a sedi- youngest MFT carries the Siwalik Group over the
mentary facies is a body of rock whose characters Quaternary deposits that include recent alluvium
bear the signature of the processes involved in its of Indo-Gangetic plains in its footwall (Hodges
formation. Mineralogical and textural characters 2000; Yin 2006 and references therein). In the TVS
of clastic sedimentary rocks and analysis of modal of the Darjiling Himalaya, the MFT is not exposed.
values of quartz (Q), feldspars (F), lithic fragments The Lower Siwalik rocks are exposed in the South
(L), monocrystalline quartz (Qm) and total lithic Kalijhora Thrust (SKT) sheet (Basak and Mukul
fragments (Lt) including polycrystalline quartz 2000). The Middle Siwalik rocks are exposed in the
(Qp ) constituting the framework of sandstones help hanging wall of several imbricate thrusts south of
to decipher the sedimentation history and the tec- the SKT and define the imbricate-type mountain
tonic setting of the rocks (Pettijohn et al 1972; front in the Darjiling–Sikkim Himalaya (Mukul
Basu et al 1975; Suttner et al 1981; Dickinson et al 2000 and references therein).
1983; Dickinson 1985; Arribas and Tortosa 2003; The Siwalik succession consists of conglome-
Datta 2005; Kundu and Matin 2007; Banerjee and rate, sandstone, siltstone and shale layers, depo-
Banerjee 2010). These established tools have also sited in the Himalayan foreland basin since the
been successfully and widely applied to under- Early–Middle Miocene (Acharyya 1976; Banerji
stand the palaeoclimate during sedimentation and and Banerji 1982; Kumar 1997; DeCelles et al 1998;
the provenance of Siwalik (Critelli and Ingersoll Brozovic and Burbank 2000; Sharma et al 2002;
1994; DeCelles et al 1998; Najman and Garzanti Yin 2006; Ranjan and Banerjee 2009). On the basis
2000; White et al 2002) and other sedimentary of biostratigraphy, the Siwalik rocks are classified
rocks (Garzanti et al 1996; Najman 2006; Ullah into three broad subgroups as Lower, Middle and
et al 2006) from different parts of the Himalaya. Upper Siwalik subgroups (Medlicott 1864) and fur-
However, such studies are not attempted from the ther subdivisions of these subgroups were done on
Darjiling Himalaya, particularly from an imbricate- the basis of mammalian fossil assemblage (Pilgrim
type frontal setting as seen in the frontal Darjiling 1913; Prasad 2001). The Siwalik sequence is now
 Middle Siwalik sediments, Tista valley, Darjiling

Figure 1. Location maps and sections: (a) A generalized map of the Himalayan orogen (after Yin 2006). (b) A simplified cross-section across the Himalaya showing major
thrusts and litho-tectonic units (after Mukul 2010). (c) Detailed map of the Tista River section with a part of the area of study shown (in broken rectangle). (d) Part of
Tista River section showing the imbricate thrust zone deforming the Middle Siwalik Subgroup. (e) Restored cross-section of Middle Siwalik Subgroup in the study area. The
shaded area is the facies association Fa3 (the lowermost Fa3, which the third facies association from the base of the lithostratigraphic column), used as the marker horizon.
75
76 Abhik Kundu et al

formally known as the Siwalik Group (Kumar et al are of two types, viz., matrix-supported and clast-
2003) and is subdivided into: (1) Lower Siwa- supported. They are common in the upper and
lik Subgroup comprising an upward-coarsening lower parts of the sequence. The sandstone beds
mudrock–sandstone succession; (2) Middle Siwalik occur throughout the sequence. They are coarse- to
Subgroup consisting mainly of sandstones, and medium-grained, medium-grained and fine-grained
(3) Upper Siwalik Subgroup consisting of conglom- sandstone; fine-grained and medium-grained sand-
erates, sandstones and mudrocks (Kumar et al stone beds dominate over the coarse- to medium-
2003). In the Darjiling Himalaya, the Lower, grained sandstone beds. Siltstone and shale are
Middle and the Upper Siwalik successions are present in the entire sequence except the lower
represented by Chunabati Formation, Geabdat part. Heterolithic units consisting of intercalated
Sandstone and Parbu Grit and Murti Boulder layers of fine-grained sandstone, siltstone and shale
Beds, respectively (table 1) (Acharyya 1994; are present in the middle and upper parts of
Kumar 1997; Matin and Mukul 2010). Accord- the sequence. Beds with sheet geometry without
ing to Banerji and Banerji (1982), the tripar- internal primary structure, planar tabular cross-
tite divisions of the Siwaliks is not applicable in strata, trough cross-strata and channel structures
the TVS in the Darjiling Himalaya (figure 1c) as are common sedimentary structures in both con-
the Upper Siwalik Subgroup is absent here. The glomerate and sandstones. Finer rocks are planar-
folded sequence of the Lower Siwalik Chunabati bedded and often internally laminated. Some of the
Formation is transported over the Middle Siwalik heterolithic units preserve soft-sediments deforma-
Geabdat Sandstone by the SKT (Basak and tion structures that may be interpreted as seismites
Mukul 2000; Mukul 2000). The Geabdat Sand- (Kundu et al 2011).
stone have been repeated by imbricate faults in
the footwall of the SKT and are exposed all
the way to the mountain front (Mukul 2000;
figure 1d, e) and accommodate a minimum of 47% 3.1 Petrography of Middle Siwalik sandstone
(3.5 km) shortening (table 2). The Siwalik Group and finer rocks
of rocks are exposed in the Tista River section 3.1.1 Coarse- to medium-grained sandstone
and adjacent road sections between the Kalijhora
(26◦ 55 30.44 N, 88◦ 27 16.39 E) in the north and The framework clasts consist of quartz, potash
mountain front (26◦ 52 52.30 N, 88◦ 28 22.69 E) in feldspar, plagioclase and rock fragments and a
the south (figure 1c). small amount of muscovite and biotite (altogether
constituting <5% of the total clasts). Size of clasts
varies between 0.25 and 0.75 mm with a dominant
size range from 0.25 to 0.6 mm. The clast-matrix
ratio varies from 4:1 to 9:1. Quartz clasts constitute
3. Middle Siwalik Subgroup ∼60–75% of the framework and dominantly com-
prise monocrystalline grains (∼90% of the quartz
The Middle Siwalik Subgroup occupies the major grains) with subordinate amount of polycrystalline
part (∼70%) of the frontal TVS south of the MBT; grains. These quartz clasts are angular to sub-
the rest is occupied by the Lower Siwalik Subgroup. rounded with rough to smooth grain boundaries.
The Middle Siwalik lithologic units are repeated Polycrystalline grains contain three or more than
several times by imbricate thrust faults. There are three individual crystals with either straight or
totally eight in-sequence and one out-of-sequence sutured inter-crystal margins. Feldspar clasts con-
imbricate thrusts (figure 1d and e). From hinter- stitute ∼30–10% of the framework. About 90% or
land to foreland, the in-sequence thrusts are named more of the feldspar grains are potash feldspar.
T1–T8, T1 being the oldest in the sequence of These are angular to subrounded and show a
thrusting and T8 is the youngest. From struc- greater degree of roundness than the quartz grains.
tural and sedimentological data, a restored cross- Feldspar grains show incipient to high degree
section (figure 1d and e) of the deformed Middle of alteration to sericite (figure 3a). Rock frag-
Siwalik succession has been prepared (Kundu et al ments constitute ∼10–15% of the framework and
2011). The most complete sequence (figure 2) is consist of fragments of gneiss, shale, phyllite and
preserved in the T3 sheet in the TVS (figure 1d mica schist. Matrix constitutes ∼20–10% of the
and e) on the basis of which we conclude that volume of rock and is made up dominantly of sil-
the minimum thickness of the Middle Siwalik sec- ica. Tiny biotite grains are also present within the
tion was ∼325 m. About 69.5% (226 m) of the matrix. Silica cement is commonly present (figure
325-m thick sequence is occupied by sandstone 3a) while carbonate (calcite) cement is occasion-
beds, ∼19.5% (63 m) by siltstone–shale and ∼11% ally present. Secondary calcite are present along
(36 m) by conglomerate beds. The conglomerates microfractures (figure 3a).
 Middle Siwalik sediments, Tista valley, Darjiling 77
Table 1. Stratigraphy of the Siwalik Group in the Darjiling–Sikkim sector of Himalayan Foreland Basin (after Matin and
Mukul 2010).

Geologic time Group Subgroup Formation and lithology

Late Miocene Upper Siwalik Subgroup Murti Boulder Bed

to Pliocene (Crude immature conglomerate)
Parbu Grit (Pebbly sandstone
and coarse-to-medium sandstone)
Siwalik Group Middle Siwalik Subgroup Geabdat Sandstone (Medium-to
coarse-grained sandstone and shale,
pebble beds and marl)
Early–Middle Lower Siwalik Subgroup Chunabati Formation (Fine-to
Miocene medium-grained sandstone, siltstone,
mudstone, marl and conglomerate)
Upper Permian Gondwana Group Damuda Subgroup Sandstone, carbonaceous shale and coal
Lower Permian Rangit pebble-shale (Talchir?) Diamictite, rythmite, quartzite marl
Precambrian Daling Group Buxa Formation
Reyang Formation
Daling Formation
Paro Group Parametamorphites with migmatitic
and foliated granitic gneiss
Darjiling Gneiss Two-mica migmatitic gneiss

Table 2. Length and percentage of shortening of base of Feldspar grains are partially altered though almost
Middle Siwalik Subgroup in the Tista River Section.
unaltered (figure 3c) grains are also present. Rock
Line Shortening fragments constitute ∼15% of the framework and
consist of mica schist, chert and shale (figure 3b
Total length of the base of 3.875 km
and c). The matrix occupies <10% of the vol-
deformed section (ld )
ume, but locally the volume of matrix is ∼20%. It
Total length of the base of 7.345 km
is mainly made up of silica with occasional pres-
restored section (lr )
ence of mica grains. Recrystallized silica cement is
Total length shortening of the 3.47 km
present along the interganular spaces (figure 3b).
base of restored section (lr − ld )
Shortening in % {(lr − ld )/lrr } × 100 47.24%

3.1.3 Fine-grained sandstone

3.1.2 Medium-grained sandstone The framework clasts consist of quartz, biotite and
muscovite (figure 3d). The size of the clasts varies
The framework clasts of these sandstones consist from ∼0.125 to ∼0.25 mm. Angular to rounded
of quartz, alkali and plagioclase feldspar, lithic quartz clasts occupy ∼80% of the framework. The
fragments and little amount of biotite and mus- quartz clasts are monocrystalline grains with either
covite. The size of clasts varies from 0.2 to 0.5 mm. smooth or irregular grain boundaries. The matrix
The clast matrix ratio is ∼9:1. Quartz clasts con- is made up of very fine sand to silt-sized quartz
stitute ∼70% of the framework. These are dom- and opaque minerals. The cement is opaque and
inantly (≥80% of total quartz clasts) monocrys- possibly iron-oxide.
talline with subordinate amount of polycrystalline
grains and are angular to subrounded with smooth
or rough grain boundaries (figure 3b). Polycrys- 3.1.4 Siltstone and shale
talline grains are with 3 or >3 individual crystals
having sutured inter-crystal boundaries (figure 3b). The siltstone and shale units consist of clasts of
Feldspar clasts constitute ∼15% of the total frame- quartz, mica and opaque minerals. Quartz grains
work. These are mainly (∼70%) alkali feldspar with are angular to rounded. Some of the shale–siltstone
subordinate (∼30% of total feldspar) plagioclase samples show a foliation that is defined by a pre-
feldspar grains. Feldspar grains are subrounded ferred orientation of the mica flakes and the long
and commonly possess smooth grain boundaries. dimension of the inequant quartz grains (figure 3e).
78 Abhik Kundu et al

quartz with either incipient or intense recrystalliza-

tion (figure 3a). Kinks in twin lamellae of plagio-
clase feldspar grains (figure 3h) and in mica flakes
is a common feature in almost all samples.

3.2 Modal analysis

Composition of the sand has a close relationship
with the composition and tectonic setting of the
source rock and the tectonic setting of the source
rock and hence the type and tectonic setting of the
source can be revealed from detrital modes of sand
and sandstones (Suttner et al 1981; Dickinson et al
1983; Dickinson 1985). The modal counts of frame-
work grains (quartz, feldspars and lithics) from 19
samples collected from different stratigraphic lev-
els are plotted to understand the type and tec-
tonic setting of the provenance (table 3). From
the QFL plot (figure 4a), recycled orogenic prove-
nance is envisaged whereas in the QmFLt diagram
(figure 4b) all points except two suggest ‘quartzose
recycled’ provenance. One of the sample plot lies on
the boundary between quartzose recycled field and
mixed provenance field while the other plots lie on
the boundary between quartzose recycled and tran-
sitional field (cf., Dickinson et al 1983; Dickinson
1985). When plotted in quartz-feldspar-rock frag-
ment (Q-F-RF) ternary diagram of Suttner et al
(1981), the plots suggest plutonic–metamorphic
source in humid palaeoclimate (figure 4c).

4. Sedimentary facies

4.1 Facies units

A detailed description of facies units is presented
Figure 2. Stratigraphic column of the Middle Siwalik Sub- in Kundu et al (2011). Conglomerate units are
group in the Tista River section showing the facies associa-
tions (Fa1, Fa2, Fa3 and Fa4 are facies associations).
divided into four different facies on the basis of the
lithological and primary structural characters.
(i) diamictite facies (figure 5a) (F1)
(ii) thick bedded conglomerate facies (figure 5b)
3.1.5 Deformation signatures in mineral grains (F2)
The Middle Siwalik sequence is faulted. The rocks (iii) crude cross-stratified conglomerate facies
in the fault zones and in the fault-related damage (figure 5c) (F3)
zones show signatures of cataclasis (figure 3f) and (iv) conglomerate facies with channel structure
deformation by frictional sliding. Rocks other than (figure 5d) (F4)
the fault rocks also preserve some grain-scale defor- Sandstones are divided into four different facies
mation features suggesting deformed nature of the
source rocks. Many quartz and feldspar grains show (i) thick bedded sandstone facies (figure 5e) (F5)
undulose extinction (figure 3a and g), whose inten- (ii) tabular cross-stratified and planar laminated
sity varies from grain to grain. Intracrystalline sandstone facies (figure 5f) (F6)
deformation bands are present in quartz grains (iii) sandstone with channel structure (figure 5g)
(figure 3g) (cf. Blenkinsop 2002). In many samples, (F7)
more than 50% of the quartz grains show undulose (iv) planar laminated fine-grained sandstone facies
extinction. Many rock fragments are recrystallized (F8)
 Middle Siwalik sediments, Tista valley, Darjiling 79

Figure 3. (a) Photomicrograph of a coarse- to medium-grained sandstone. Q – quartz showing undulose extinction by dislo-
cation glide, Qp – quartz showing subgrains with grain boundary migration indicating dislocation creep, Rq – rock fragment
(recrystallized quartz), R – rock fragment (schist), F – feldspar partially altered to sericite, S – silica cement, C – sec-
ondary carbonate filling microfractures. (b) Photomicrograph of a medium-grained sandstone. Q – monocrystalline quartz,
P – polycrystalline quartz, C – irregular and crenulated grain boundary of quartz, R – rock fragment (chert), S – silica
cement. (c) Photomicrograph of a medium-grained sandstone. Q – quartz, F – fresh grain of plagioclase feldspar, Sh – shale
(rock fragment) and C – chert (rock fragment), S – recrystallized silica cement. (d) Photomicrograph of a fine sandstone.
Light-coloured grains are quartz, darker (brown) grains are mica. (e) Photomicrograph of a siltstone–shale sample. Note
the alignment of inequant quartz grains following the preferred alignment of mica grains (marked by dotted line). (f ) Pho-
tomicrograph of a cataclasite consisting T – transgranular, I – intragranular and C – circumgranular micro-fracture. Curved,
unbranched and branched micro-fractures are also evident. (g) Photomicrograph of a sandstone consisting of quartz with
undulose extinction (U) and intracrystalline deformation band in quartz (D). (h) A sandstone containing plagioclase grain
(K) with kinked twin lamellae.
80 Abhik Kundu et al
Table 3. Modal values of monocrystalline quartz (Qm), polycrystalline quartz (Qp), feldspars
(F) and lithic fragments (L) in Middle Siwalik sandstones.

Sample Qm Qp F L

17A 68.57 4.80 9.13 17.5

17FW 65.47 14.38 9.82 10.33
19 59.00 8.70 11.90 20.4
28 66.00 7.70 12.60 13.7
23 62.20 10.30 7.10 20.4
28A 49.20 15.90 12.60 22.3
4 55.00 11.00 16.40 17.6
4a(3) 58.30 9.00 11.70 21
HST–HW 59.20 7.90 15.60 17.3
13D/1 63.30 6.60 15.60 14.5
18 50.00 13.20 14.50 22.3
20-1 49.00 10.00 21.00 20
27A 50.60 15.60 14.80 19
6/1 59.30 6.90 15.60 18.2
14 57.60 11.10 15.70 15.6
13 61.60 7.60 13.30 17.5
13c 54.60 16.05 15.75 13.6
AJST–FW1 65.30 4.60 16.90 13.2
13D/1 65.30 11.30 10.20 13

Figure 4. (a) Ternary plot of Q–F–L modal values of Middle Siwalik sandstones in the recycled orogenic provenance
field (after Dickinson 1985). (b) Ternary plot Qm–F–Lt modal values in the quartzose recycled provenance field (after
Dickinson et al 1983). (c) Ternary plots of Q–F–RF modal values indicate to plutonic-metamorphic provenance within
humid palaeoclimate (after Suttner et al 1981).

Finer-grained rocks are present either as hete- lithic facies (F9), while the siltstone–shale units
rolithic units or as siltstone and shale units. are addressed as siltstone–shale facies (F10).
Heterolithic units consisting of shale–siltstone– Attributes of different facies are tabulated for
fine–sandstone layers are addressed as hetero- ready reference (table 4).

Figure 5. (a) Exposure of diamictite. Pen (15 cm) for scale. (b) Thick-bedded, clast-supported conglomerate (Cgl) with
normal grading. Note that the clasts are dominantly subrounded with moderate to high sphericity. Broken line marks
erosional contact with underlying sandstone (Sst). Hammer (45 cm) for scale. (c) Conglomerate with low angle planar
tabular cross-strata (marked by dotted lines). Traces of bedding planes are marked by broken lines. Pen (15 cm) for scale.
(d) Sharp contact (marked by arrow) between matrix-supported conglomerate (C) and coarse- to medium-grained sandstone
(S). The conglomerate shows interlaced trough cross-stratification (marked by broken line). Hammer (45 cm) for scale.
(e) Sandstone with sheet geometry. Bedding planes are marked by broken lines. Height of the person standing is 155 cm.
(f ) Medium-grained sandstone with plane laminae (P) and cross-strata with asymptotic base (C). Bedding plane is marked
by broken line. (g) Interlaced channels in sandstone. B – bedding planes, C – channel sand. Hammer (45 cm) for scale.
Middle Siwalik sediments, Tista valley, Darjiling 81

.
 82

Table 4. Characteristics of different facies units in the Middle Siwalik Subgroup (after Kundu et al 2011).

Facies Rock type Attributes Interpretation

F1 Diamictite • Thick beds (1–2.2 m) Deposition on a gentle slope from mudflow in proximal-fans
• Angular to rounded pebbles and granules or in proglacial outwash zones (Maizels 1993; Nemec and
• Grading commonly absent, locally both normal and Postma 1993; Zielinski and van Loon 1999a, 2000; Neves
reverse grading et al 2005)
• Plane stratification defined by arrangement of pebbles
High volume (>50%) of sandy–muddy matrix
F2 Thick-bedded • Thick (∼2 to 5 m) beds showing sheet geometry Sedimentation from ice-melt water in low sinuosity streams
conglomerate • Framework consists of angular to subrounded pebbles (Rust 1975; Gordon and Bridge 1987; Miall 1996; Zielinski
• Concentration of pebbles varies vertically and laterally and van Loon 1999a)
The conglomerates are matrix-supported
F3 Conglomerate with • Comparatively thinner beds than others Deposited during migration of bars under high energy flow
of crude cross-strata • Angular to subrounded pebbles glacial melt water in straight or braided streams within
• Planar tabular cross-strata, defined by arrangement mid- or distal-fan setup (Reineck and Singh 1980; cf. Nilsen
of pebbles 1982; Opluštil et al 2005)
These conglomerates are clast supported
Abhik Kundu et al

F4 Conglomerate with • Bed thickness 0.75 cm to ∼3 m Migration of dune (Opluštil et al 2005) and shifting of stream
trough cross-strata • Subrounded to rounded clasts channels accompanied by channel filling (Miall 1977) or by
• Normal grading/no grading vertical aggradations of braided streams (Casshyap and
Trough cross-stratification Tewari 1984) typical in the mid-fan part
F5 Thick-bedded • Bed thickness 60 cm to ∼1 m Deposition from sheet-flood after waning of energy (e.g.,
sandstone • Sheet geometry Heward 1978; Demicco and Gierlowski 1986; Zavala 2008).
• Medium- to coarse-grained, occasionally pebbly Presence or absence of channelized deposits indicates either
• Pebbles are angular and are haphazardly arranged in mid-fan or distal-fan setup (Blair and McPherson 1994;
sandy–muddy matrix MacCarthy 1990; Sadler and Kelly 1993)
• Show normal grading
Internal primary structures are almost absent
Table 4. (Continued).

Facies Rock type Attributes Interpretation

F6 Tabular cross stratified • Bed thickness up to 1 m Down the current bar and dune migration in low sinuosity stream
and horizontal • Medium-grained sandstone channels in mid-fan setup (Miall 1992, 1996; Cadle and Cairncross
laminated sandstone • Normal grading 1993; Zielinski and van Loon 1999b and references therein) or
• Bedding parallel lamina, tabular cross-strata deposition in bars adjacent to channel banks (cf. Mazumder 2002)
often with asymptotic base
F7 Sandstone with channel • Bed thickness up to ∼1 m Deposition in a laterally coalescing stream channel system in the
structure • Medium to coarse-grained sandstone mid-fan setup (Tewari 1995; Opluštil et al 2005). Channel floors
• Interlaced channel structures and trough cross-strata filled with pebbles indicate high-energy conditions in the proximal-
• Channels are of various dimensions fan zone (Fraser 1982; Zakir Hossain et al 2002)
• Bases of channels are often pebbly Facies F5, F6 and F7 suggest deposition within a braided river
system in mid-fan setup (cf. Friend et al 1976; Kumar et al 2004;
Scherer and Lavina 2006; Veiga and Spalletti 2007)
F8 Fine-grained sandstone • 10 to 35 cm thick beds Facies F8, F9 and F10 were deposited under lower flow conditions
with horizontal • Plane laminated (Rust 1972; Cant 1978) from suspension load in shallow braided
lamination • Associated with shale and coarser rocks channels in distal-fan (Hartley 1993; Dreyer 1993) or are overbank
deposits (Kumar and Tandon 1985; cf. Nichols and Fisher 2007)
F9 Fine-grained sandstone– • Thickness up to 60 cm
siltstone–shale • Mainly associated with shale and fine-grained sandstone
(heterolithic facies) Storehouse of various soft-sediment deformation structures
F10 Siltstone and shale • Thickness up to 30 cm
Middle Siwalik sediments, Tista valley, Darjiling

• Associated with all types of rocks

• Often as capping of channel structures in sandstones
and conglomerates
83
84 Abhik Kundu et al

4.2 Sequence of facies associations units. The overlying association of 37 m thickness

consists of F5, F7, F8, F9 and F10 and is devoid of
The present work includes a comprehensive any conglomerate unit. This association is similar
account on associations of facies units and rep- to Fa2 and is thus named as Fa2. Upward, an asso-
etition of the facies associations in the Middle ciation of 25 m thickness comprising F6, F8 and
Siwalik sequence to understand the depositional F9 with a single F4 conglomerate unit forms an
environment and also aims to construct a broad association similar to that of Fa3. Overlying this
stratigraphic sequence of the Middle Siwalik is an 80-m thick pile of sandstone and finer sedi-
Subgroup exposed in the TVS of the Darjiling mentary rocks. The member facies are F5, F7, F8,
Himalaya. Within a particular broad depositional F9 and F10 with the coarser members dominating
setup energy conditions may vary in different seg- the lower portion of this association. The member
ments of the basin and also in different seasons. facies are similar to Fa2 and the association is thus
A facies association is a product of a series of regarded as Fa2. The next upward association con-
sedimentary processes and interactions between sists of F3, F4 and F6, without any siltstone–shale
them depicting a particular sedimentary envi- member. This association occupies a thickness of
ronment (Neves et al 2005; Wysocka 2009). A 30 m and the member facies are similar to Fa1.
facies sequence is the order of occurrence of facies The top 20 m of the sequence consists of a facies
or facies associations which have developed in association similar to Fa2. The Middle Siwalik
response to a repetitive series of processes due sequence in the TVS thus indicates that each facies
to regular changes in conditions of sedimentation association appears more than once in the strati-
(Nichols 2009 and references therein). Hence, facies graphic column. Hence, the 325 m thick lithostrati-
associations and sequences help to infer the sub- graphic column of the Geabdat Sandstone in the
environments within a larger depositional setup. TVS is dominated by sandstone units. Conglomer-
In the studied section of Middle Siwalik Subgroup, ate and siltstone–shale units are intercalated with
every facies appears a number of times and the the sandstone units. Conglomerates appear spo-
sequence shows different facies associations from radically and is almost absent in the middle part
the bottom to the top. The associations and their of the sequence while the siltstone–mudstone units
repetitions are identified for the interpretation of are distributed throughout the sequence excepting
the depositional setup. the lower part.
In the TVS, the lowermost facies association
is 30-m thick and is exposed in T3, T7 and
T8a thrust sheets (figure 2). This association con-
sists of conglomerate and sandstone units of facies 5. Interpretation
F3, F4 and F6. In this part of the sequence,
any facies of sub-sand grain-size is absent. This
5.1 Depositional environment
facies association is addressed as Fa1. The next,
44 m upward in the sequence from Fa1, the A summary of interpretation of the facies units is
Fa2 sequence consists of an association of facies presented in table 4. The cross-bedded conglomer-
F5, F7, F8 and F9. In this association, coarse- ate and sandstone in the associations Fa1 and Fa2
to medium-grained sandstone, shale and siltstone point to deposition from high-energy channelized
appear for the first time, while the conglomer- flow. The laminated sand and finer sediments sug-
ate is absent. Upward in the sequence, the next gest deposition from overbank flows. Hence, these
facies association, Fa3 is dominated by medium- facies associations point to current and surface
grained sandstone and finer-grained rocks. This flood domination in high-energy regime and thus
association is 30-m thick, begins with F6 and is possibly indicate deposits of the mid-fan zone (c.f.,
dominated by this facies with one intercalated Mishra et al 2004; Wysocka 2009 and references
F4 conglomerate while the upper half consists therein). Presence of interlaced channel structures
of intercalation of F8, F9 and F10. This facies in sandstones bodies are indicative of the braided
association can be identified in all thrust sheets nature of stream channels typical for the mid-fan
in the Middle Siwalik and therefore it is used setup, though possibility of proximal-fan environ-
as the marker horizon for cross-section balancing ment cannot be fully ruled out as in-fills of coarser
(figure 1d and e), following the area balance grain size indicate much higher energy condition of
method (Marshak and Mitra 1988). The next facies deposition characterizing a proximal fan environ-
association, Fa4, comprises F1 and F2 conglome- ment. This is possible in tectonically active setup
rate units and finer members namely F8, F9 and of foreland basins of a fold and thrust belt where
F10 and occupies ∼25 m thickness above the repetitive thrust front propagation towards fore-
underlying Fa3. This Fa4 is completely devoid of land may bring proximal fan environment over
medium- or coarse- to medium-grained sandstone existing mid- or distal fan zones. Fa3, F8, F9 and
 Middle Siwalik sediments, Tista valley, Darjiling 85

F10 facies indicate waning energy depositional con- 1999a). This interpretation is supported by the fact
ditions. Finer sand and shale of Fa3 are possibly that the Mio–Pliocene time was a period of intense
deposited within shallow braided streams in dis- glaciation on Himalayas (Gupta et al 2000 and
tal fan setup (Mishra et al 2004 and references references therein). From the sequence of facies
therein). Facies package in Fa4, is dominantly con- associations (figure 2), it is evident that each
glomeratic and appears to have been formed by association reappears with time reflecting spatial
high-energy gravity flow, typical to the proximal oscillation of proximal-, mid- and distal fan with
zone of an alluvial fan system (Mishra et al 2004; time within an alluvial fan setup. This may be a
Neves et al 2005; Nichols and Fisher 2007 and result of a combination of foreland-ward migration
references therein, Wysocka 2009 and references of the mountain with propagation of Himalayan
therein). Diamictites (F1) in Fa4 facies association thrusts (Raiverman 2002) alternate with moun-
might have deposited within a proglacial outwash tainward shift of the fan-apex due to erosion during
zone (c.f., Maizels 1993, Zielinski and van Loon tectonically quiet phases (figure 6).

Figure 6. Schematic diagram showing palaeogeographic and tectonic setup of the Middle Siwalik sedimentation in the TVS.
(a) Formation of alluvial fan at the foreland basin at the initial phase (Fan1). (b) Migration of the fan (to the new phase
Fan 2), basin ward shift of fan apex with foreland ward propagation of thrust and uplift of the hinterland. (c) Erosion
at mountain front and adjacent hinterland in the tectonically quiet phase, migration of fan (Fan 3) with hinterland ward
retreat of fan-apex. (d) Migration of the fan (to the new phase Fan 4), basin-ward shift of fan apex with foreland-ward
propagation of thrust and uplift of the hinterland.
86 Abhik Kundu et al

5.2 Provenance and palaeoclimate sources of the sediments. Mineral character of the
clasts and plots of modal values suggest that sedi-
Modal values of framework minerals suggest recy- ment transport and deposition took place in humid
cled orogenic source for the sediments constitut- palaeoclimate. Analysis of sedimentary facies and
ing the Middle Siwalik Group rocks (figure 4a and sequence of facies associations reveal recurrence of
b). Metamorphic or mixed metamorphic-acid plu- proximal-, mid- and distal fan depositional condi-
tonic rocks may be the sources of sediments con- tions within an alluvial fan setup.
taining monocrystalline quartz showing undulose
extinction and polycrystalline quartz (Pettijohn
et al 1972). Coexistence of quartz and feldspar Acknowledgements
grains with variable intensities of undulose extinc-
tion, quartz grains with intracrystalline deforma- Abhik Kundu acknowledges University Grants
tion bands, plagioclase grains with deformed twin Commission, India for the research grant PSW-
lamellae and warped and kinked mica flakes posi- 22/06-07 (ERO). He also acknowledges supports
tively point to deformed source rocks. Character from Dr D K Kar, Principal, Asutosh College. Abdul
of the rock fragments in the framework points to Matin acknowledges funding from DST, Govt. of
metamorphic hinterland-like gneiss in the Greater India (Grant No. DST No. SR/S4/ES:276/2007
Himalayan Crystalline terrain and quartzite, phyl- dated 04.06.2010). Malay Mukul acknowledges
lite and muscovite–biotite schist in the Daling funding from IIT-B (Grant No. 09IRCC017). The
Group of Lesser Himalaya (Raina 1976; Yin 2006 authors thank Prof. Talat Ahmad and the anony-
and references therein; Bhattacharyya and Mitra mous reviewers for their constructive comments
2009; Ranjan and Banerjee 2009). and suggestions that helped to improve the paper.
Plots in the Q-F-RF diagram (figure 4c) of
Suttner et al (1981) suggest that plutonic and
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MS received 15 October 2010; revised 26 September 2011; accepted 1 October 2011