Journal of the Geological Society, London, Vol. 158, 2001, pp. 151–162. Printed in Great Britain.
Sedimentology of the Indus Group, Ladakh, northern India: implications for the
timing of initiation of the palaeo-Indus River
1
H. D. SINCLAIR 1 & N. JAFFEY 1
Department of Geology and Geophysics, Grant Institute, University of Edinburgh, West Mains Road,
Edinburgh, EH9 3JY UK (e-mail: [email protected])
Abstract: The upper reaches of the Indus River form a longitudinal network that drains a large portion of
the western Himalaya. It plays an important role as a sediment routing system, feeding the Indus Delta
and submarine fan, and has played a controlling role in the denudational history of the western Himalaya.
Given the rivers long-term significance, its timing of initiation remains poorly constrained. The facies,
palaeocurrents and provenance of the post-early Eocene Indus Group preserved along the upper reaches
of the modern Indus reveal a history of fluvial/deltaic and deep-water sedimentation in an intermontane
basin dominated by internal drainage. Hence, the Indus Group is not considered to represent the deposits
of a palaeo-Indus River as previously thought. Illite crystallinity values and apatite fission track dating of
the Indus Group suggest that the succession was buried to temperatures of 155–280C, and that unroofing
started in early Miocene times and proceeded at 0.1–0.4 mm a1 to the present. The youngest sedimentary
rocks preserved along the Indus Basin are early Miocene in age from the tectonostratigraphically
equivalent deposits around Kargil to the west of the study area. The transition from sediment
accumulation to erosional unroofing in early Miocene times coincides with accelerated regional unroofing
of the High Himalayas to the south, and the initiation of the Indus Delta/submarine fan to the west.
Differential uplift between the northward thrusting of the Zanskar and Indus sedimentary succession
against the undeformed Ladakh Batholith provides a mechanism for post-early Miocene initiation of a
major longitudinal river. Hence, early Miocene times is believed to represent the earliest possible age of
initiation of the palaeo-Indus river following this course.
Keywords: Indus, Himalayas, Ladakh, molasse, intermontane basins.
The Indus Basin of Ladakh, northern India, records a history
of sedimentation along the Indus Suture Zone that spans
accumulation in a fore-arc basin prior to collision, and an
intermontane basin post-collision (i.e., from Early Palaeocene
to Miocene times; Garzanti & Van Haver 1988; Searle et al.
1990). The younger, fluvially dominated sedimentary succession termed the Indus Group are preserved along the presentday course of the Indus River, and have been cited as evidence
for a long-lived, antecedent history of development of the
Indus system along this route (Searle 1996; Qayyum et al.
1997; Friend 1998; Clift et al. 2000). New facies and palaeocurrent data from the Indus Group are presented in order to
evaluate this proposal.
The modern Indus River represents one of the major rivers
of the world, with the seventh highest sediment discharge of
291 Mt a1 from its river mouth (Milliman & Meade 1983;
Hovius 1998). It is sourced from the southern margin of Tibet
in the region of Mount Kailas, from where it flows westnorthwestward, for approximately 880 km along the strike
of the mountain belt (Fig. 1). It initially follows the
Indus–Tsangpo continental suture zone, then traverses the
Ladakh batholith, finally heading south, cutting through
the Himalayan mountain belt around the Nanga Parbat
Massif (Fig. 1). This distance of longitudinal flow is second
only to the Yarlung/Tsangpo river which follows a similar
tectono-geomorphic route to the eastern syntaxis of the
Himalaya before heading southward as the Brahmaputra.
Large-scale longitudinal rivers form the first harmonic of
topography in large mountain belts (Koons 1994). Their
development significantly modifies sediment routing systems
compared to the transverse networks found in many smaller
mountain belts (Hovius 1996). The most spectacular example
of this is south of the Himalaya where the Indus and Bengal
fans are fed by river systems running longitudinally within the
mountain belt (Indus and Yarlung–Tsangpo), and along the
mountain front (Ganges and Brahmaputra; Fig. 1).
Longitudinal rivers may also play an important role in the
denudation of a mountain belt. The longitudinal component of
river systems are typically long, low gradient alluvial channels
(i.e., channels bound by their own alluvial sediment as opposed
to bedrock), and where fluvial incision rates, and hence
denudation rates, are low compared to topographic uplift
(Koons 1995). However, where these rivers transverse the
Fig. 1. Map of main Himalayan rivers feeding the Indus and Bengal
fans. The study area is boxed, and is located along the longitudinal
portion of the Indus River in Ladakh, northern India (Fig. 3).
151
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H. D. SINCLAIR & N. JAFFEY
Fig. 2. Comparison of proposed
stratigraphies for the Tertiary of the
Indus Basin, Ladakh. Wakka Chu
Molasse shown but not correlated.
mountain range, high discharge combined with steep slopes
results in high rates of fluvial incision. Modelling suggests that
high fluvial incision rates will generate accelerated regional
denudation through the maintenance of hillslopes by masswasting (Anderson 1994); these models are supported by
comparisons of incision rates with longer-term denudation
rates where the Indus River flows around the Nanga Parbat
massif (Burbank et al. 1996). Additionally, it has been suggested that these sustained high rates of denudation may have
resulted in the focusing of increased mechanical strain in
this area as evidenced by folding and thrusting (Seeber et al.
1998; Zeitler & Koons 1998). Given the sedimentological,
denudational and possible mechanical significance of the Indus
River as a major longitudinal system, it is important to have
some knowledge of the timing of its initiation, and hence to be
able to compare it to the sedimentary record in the Indus fan,
and the erosional record of the Nanga Parbat Massif.
The natural location to look for evidence of a palaeo-Indus
River running longitudinally within the mountain belt is along
the course of the present-day river. Three localities along the
upper Indus preserve a sedimentary record (Fig. 1): (1) Mount
Kailas in southern Tibet where thick conglomerates drape the
southern slopes of the Gangdese Batholith (Gansser 1964;
Harrison et al. 1993); (2) Ladakh, northern India (this study);
and (3) Kargil, NE Kashmir (Brookfield & Andrews-Speed
1984b). The Indus Group in Ladakh was chosen for this study
due to the presence of previous background work (Van Haver
1984) and logistical considerations. In order to test the
presence or absence of a palaeo-Indus within the Indus
Group, testable predictions must be made. During periods of
aggradation of a palaeo-Indus River along the Indus suture
zone, facies and palaeocurrent data should record evidence of
sustained fluvial discharge to the NNW. During aggradation,
the denudation of the drainage basin, and the discharge of
sediment out of the system would have been relatively low. In
contrast, during periods of relatively high denudation and
sediment discharge, the fluvial system would be degradational
and incising, and the only preserved evidence may be an
unconformity. During these periods, the thermochronology of
the rocks beneath the unconformity may be used to evaluate
the timing of initial degradation. This record may be complicated by localized, structurally induced uplift and degradation during sedimentation; this would be recorded by growth
strata and time equivalent unconformities. These predictions are tested using the sedimentology, stratigraphy and
thermochronology of the Indus Group.
The Indus Group, Ladakh
The Indus Group has been defined by Searle et al. (1990), and
represents a dominantly continental succession, at least 2 km
thick, of post-early Eocene age. It crops out SW of the Indus
River between Khalsi in the NW, and Upshi in the SE.
Underlying the Indus Group are a succession of marine strata
of Late Albian to Eocene age described by Van Haver (1984)
and Brookfield & Andrews-Speed (1984a). Structurally, the
Indus Group are located in a 15–20 km wide zone, bound to
the SSW by a suite of highly deformed rocks derived from the
Indian passive margin (Zanskar units), the forearc (Nindam
Unit) and the volcanic arc rocks (Dras Unit), and ophiolitic
melange units of the Indus Suture Zone (Bassoulet et al. 1983;
Searle et al. 1990). The north-northeastern margin of the
Indus Group basin onlaps onto the Ladakh arc batholith of
the southern margin of Tibet (Van Haver 1984; Searle et al.
1997; Fig. 2). The Indus Group have been interpreted as the
remnants of an intermontane basin (Van Haver 1984; Garzanti
& Van Haver 1988; Searle et al. 1990), conformably underlain
by sediments which represent marine conditions in a fore-arc
basin (Searle et al. 1990). Intense structural deformation and
lack of preserved microfauna have hampered the establishment
of a reliable biostratigraphy for the Indus Group (Fig. 2).
However, at the base of the succession are foraminiferal
limestones of the Sumda Formation which were dated as
Palaeocene by Van Haver (1984) using gastropods and
bivalves, and have been recently dated as latest Palaeocene
INDUS GROUP SEDIMENTATION
153
Fig. 3. Simplified geological map
showing the location of the Indus Group
relative to the modern Indus River in
Ladakh. The sedimentary rocks of the
Indus Group have been thrusted and
folded in a northward direction against
the undeformed Ladakh Batholith. The
cross-sections in Fig. 4 are located.
Location of samples for apatite fission
track age determination are starred, with
their ages shown next to the locations.
(54.9 Ma; O. Green, pers. comm.) using nummulitid biostratigraphy. Higher in the succession are remains of palm
leaves and bivalves dated as Upper Eocene/Lower Oligocene
(Van Haver 1984). The uppermost strata (Nimu Formation) of
the Indus Group are undated. In the Kargil area, 80 km west
of this study area, are along strike, tectonostratigraphically
similar successions to the Indus Group called the Wakka Chu
Molasse. The youngest dates from these rocks are from the
Taramsu Formation that yields mammal remains dated as
early Miocene (Brookfield & Andrews-Speed 1984b).
Garzanti & Van Haver (1988) studied the petrography
and geochemistry of the Indus Group and underlying
successions, and demonstrated that the mid-Cretaceous to
early Eocene succession records submarine sedimentation
derived from the progressive unroofing of the volcanic
cover of the magmatic arc, now represented by the Ladakh
Batholith; the resultant sandstone composition is similar to
modern and ancient fore-arc basins. At latest Palaeocene
times (beginning Indus Group) there was a transition to
predominantly continental sedimentation, associated with
a sharp increase in plutonic detritus from the Ladakh
Batholith, along with ophiolite-derived sediment. The final
termination of marine sedimentation in early Eocene times is
interpreted to record the initiation of continental collision
(Searle et al. 1987).
Within this tectono-stratigraphic framework, the depositional history of the post-early Eocene succession is analysed
herein (i.e., the time from which no evidence of marine
sedimentation occurred). This means that the Chogdo
Formation which underlies the Nummulitic Limestones
(Searle et al. 1990; Fig. 2) is not documented. The development of a detailed lithostratigraphy within which the sedimentology can be studied requires interpretation of the
structural sections, with correlation of units between fold and
thrust packages. The assumption in building a more detailed
stratigraphic succession through such a deformed region is
that, for the most part, reverse faults and thrusts carry older
material over younger.
Structural cross-sections
Three cross-sections through the Indus Basin were aligned
SSW–NNE, perpendicular to the structural strike (Figs 3 & 4).
Multiphase folding and thrusting have resulted in the overprinting and interference between structures with vergence
to the NNE and SSW (Searle et al. 1990). The southsouthwestern boundary of the Indus Basin sediments is the
shear zone of the Indus suture, with an ophiolitic melange
exposed near Chilling (Fig. 3a). To the south of this suture is
the Lamayuru Formation which records deep-water sedimentation on the outer margins of the Indian passive margin
(Searle et al. 1990, 1997). The northern boundary of the Indus
Group is the Ladakh Batholith, which is locally onlapped by
Indus Group sedimentary rocks of the Maastrichtian Basgo
Formation; these have been interpreted as alluvial fan deposits
on the northern margin of the fore-arc basin (Garzanti & Van
Haver 1988; Garzanti et al. 1996).
The Indus Group is located in the north-northeastern
portion of these cross-sections. Each of the structural sections
can be divided into a northern and southern unit, separated by
the Choksti fault; a high-angle, south-southwestward dipping
reverse fault that transported the Chogdo Formation over the
Nurla Formation (Searle et al. 1990; Figs 3 and 5). In section
c (Fig. 4), this fault has been back-steepened to the extent that
it is now dipping north-northeastward. SSW of this fault, the
Jurutze Formation (Searle et al. 1990) forms the core of a
broad (5–15 km), complex anticlinorium, with the Sumda
Formation (Sumda Gompa Formation; Van Haver 1984),
Chogdo Formation and the Nummulitic Limestones (Van
Haver 1984; Searle et al. 1990) folded and thrusted in its limbs.
Immediately north of Chogdo, 100 m scale folds with opposing
vergence exhibit complex interference structures. Axial planar
cleavages can be seen dipping both to the NNE and SSW.
In the Zanskar Gorge (Fig. 4a), to the north of the Choksti
fault (in its footwall), is another large (3 km wavelength),
upright anticlinorium with vertical fold hinges developed in the
Nurla Formation. The northern limb again shows interference
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H. D. SINCLAIR & N. JAFFEY
Fig. 4. Structural cross-sections through the Indus and Tar groups showing some complex interference structures, but with a dominantly
northward vergence in the northern areas. Cleavage orientation is shown by the dashed lines. Measured sedimentary sections illustrated in
Figs 5, 6, 8 and 9 are located.
between upright and SSW-vergent folds on a 50–100 m wavelength. This anticlinorium is again bound to the north by
another thrust on to the Nimu Formation. Sections b and c
also show north-northeastward vergent folding and thrusting to the north of the Choksti fault, accompanied by a
progressive decrease in deformation. The thick-bedded Hemis
INDUS GROUP SEDIMENTATION
155
the Nummulitic Limestones, as it is present at the base of the
Nurla Formation in a detached thrust unit to the north. The
following descriptions and interpretations commence with the
Nummulitic Limestones which are thought to record the last
evidence of a marine seaway between the Indian and Asian
plates, and hence sedimentation along the suture immediately
prior to collision (Searle et al. 1987).
Nummulitic Limestones. The Nummulitic Limestones are best
exposed along the Zanskar Gorge, 2–4 km south of Choksti
(Fig. 4, section 1; Fig. 5). Benthic nummulitid foraminifera
have yielded an age of 54.9 Ma (Green et al. pers. comm). The
formation is approximately 180 m thick, and commences at its
base with 2–3 m thick, coarse sandstones and mudstones
interbedded with approximately 1 m thick nummulitid-rich
packstones. Many of the limestone units are sharp-based with
locally developed pebble layers; pebbles comprise micritic
limestone, quartz vein, schists and green mudstones, with no
granodiorite. Intervals of siltstone and fine sandstone show
climbing ripple lamination and low-angle cross-bedding.
Higher up the section, packstones dominate and contain a
more varied fauna with small isolated corals, oysters and other
bivalves. Details of the foraminifera present in the Nummulitic
Limestones can be found in Van Haver (1984).
The Nummulitic Limestones abruptly overlie floodplain
mudstones of the upper Chogdo Formation (Searle et al. 1990;
Fig. 5), and are here interpreted to represent a transgression
into an inner, carbonate ramp setting; i.e., they accumulated at
approximately mean fair-weather wave-base (Burchette &
Wright 1992). The packstones suggest wave reworking, but
insufficient to completely remove the lime mud component.
The sharp, pebbly bases to some units suggest episodic storm
influence, and the sandstone intervals may record sand-rich
shoals or even shoreface deposits similar to sandstone units
which intercalate with the time equivalent Nummulitic
Limestones of the Alpine periphery (Sinclair et al. 1998).
Fig. 5. Sedimentary section through the succession immediately
underlying the Indus Group, measured south of Sumda-Do, Zanskar
Gorge (Fig. 3). This succession records fore-arc sedimentation prior
to closure of the Tethys Ocean. The Nummulitic Limestones
(uppermost Palaeocene; Van Haver 1984) record the last
unambiguous evidence of marine conditions, and have been used as
an age constraint on continental collision (Searle et al 1987).
Conglomerates found in the east, are steeply tilted, but only
locally folded into large wavelength (several hundred metres)
folds.
Stratigraphy and sedimentology
The stratigraphic subdivision used here is based on Searle et al.
(1990; Fig. 2); which itself was based on Thakur (1981) and
Thakur & Misra (1984). The Jurutze, and Sumda formations
underlie the Indus Group which were termed the Tar Group by
Searle et al. (1990; Figs 2 & 5). In the southern succession of
the Zanskar Gorge (Fig. 4a), the Nummulitic Limestones are
overlain by the Nurla Formation. North of the Choksti fault,
the lower part of the Nurla Formation comprises grey
sandstone/mudstone alternations exposed at the Choksti
Bridge, and hence named the Choksti Bridge Member. It is
possible that the Choksti Bridge Member is synchronous with
Nurla Formation. The Nurla Formation has been documented
in the Zanskar Gorge (Fig. 6a) and between Shang and
Martselang (Fig. 6b). The succession in the Zanskar Gorge is
located immediately north of the Choksti Conglomerate, i.e.,
stratigraphically beneath it (Fig. 4a). It is also found further to
the north in a separate thrust sheet.
Overall, the total thickness for the Nurla Formation is at
least 700 m, comprising alternations of coarse sandstone units
from 10 to 20 m thick, with thin to medium-bedded siltstones,
fine sandstones and mudstones forming units from 8 to 20 m
thick; the overall sandstone/mudstone ratio is approximately
60%. The sandstones are sharp-based, with pebbly to conglomeratic basal deposits; the basal surfaces locally show a
10–20 disconformable relationship with the underlying
sandstone/mudstone alternations. Most units show some
degree of upward fining, and commonly terminate abruptly
upwards into the siltstones and mudstones. Bedding geometries are variable, with tabular to lensoid beds showing
low-angle cross-cutting relationships. The sandstone units contain planar and trough cross-stratification with up to 1 m thick
cross-sets. Locally, the sandstone units are interbedded with
0.5–1 m thick conglomerates. The intervening mudstones and
sandstones vary in colour from red to green to dark grey.
Siltstone beds range from 2 to 5 cm, and are normally graded.
Remains of leaves and small wood fragments are common.
Some of the red mudstones are mottled, with the localized
development of horizons rich in small (<2 cm) irregular, cream
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H. D. SINCLAIR & N. JAFFEY
Fig. 6. Sedimentary sections through the Nurla Formation: (a)
exposed along Zanskar Gorge (Fig. 4a), and (b) exposed in valley
between Shang and Martselang (Fig. 4c). These sections record the
intercalation of thick sandstone units with red, green and grey
mudstones and siltstones. The successions are interpreted as
the deposits of aggrading fluvial systems with meandering and
braided channels cutting through fine-grained floodplain deposits.
coloured, calcareous nodules. The outcrops north of
Sumda-Do (Fig. 6) comprise thinner sandstone units (1–3 m)
with extensive planar cross-bedding (30 cm cosets). Conglomerates are also better developed (up to 5 m thick), comprising well-rounded, clast-supported conglomerates with
clasts up to 15 cm across. Clasts are dominated by metasediments including pelites, grey micrites and chert pebbles,
approximately 20–30% of the clasts are granodioritic. Van
Haver (1984) also identified fragments of peridotite which may
have been derived from the ophiolitic suture zone or the
Spontang Ophiolite (Searle et al. 1997). The conglomerates
show low-angle cross-bedding. Palaeocurrent measurements
from the Nurla Formation indicate a variable southward
to south-eastward directed palaeoflow (Fig. 7a), with an
additional north-directed component (Fig. 7b).
This succession is interpreted as the aggradational deposits
of meandering and braided, sandy and pebbly fluvial channels
over floodplain muds and silts. Rapid channel avulsion processes led to the abrupt transitions from channel sandstones
and conglomerates to floodplain fines. The thinner bedded
sandstones are considered overbank flood deposits. The lowangle discordancies along bedding surfaces record broad
downcutting into underlying floodplain mudstones.
In the lower parts of the Nurla Formation near the Choksti
Bridge along the Zanskar Gorge (Fig. 4a), is a succession of
strata that are significantly different from the bulk of the Nurla
Formation. It is at least 100 m thick, and comprises thin to
thick-bedded (<1.5 m), medium to coarse sandstones, interbedded with dark grey siltstones and mudstones. The sandstones are tabular bedded, sharp-based with locally developed
load structures. The beds are non-graded to normally graded,
with ripple laminated tops. The siltstones and mudstones are
finely laminated to bedded on a 1–15 mm scale, with ripple and
planar laminae. These strata are interpreted as the result of
episodic deposition by turbidity currents in a sub-wave base
environment.
Fig. 7. Palaeocurrent roses for data from the Indus Group. The
plots are binned at 10 intervals, and the number within each
interval increase linearly from the origin. All data have been
corrected for structural dip. (a) Dip of foresets from planar
cross-bedding in the Nurla Formation, south of Stok Palace.
(b) Plan-view ripple forms and planar cross stratification data from
the Nurla Formation, Zanskar Gorge. (c) a-axis pebble imbrication
data from the Choksti Conglomerate Formation, Zanskar Gorge
(only pebbles with all axes well exposed were measured). (d) Planar
cross-stratification and a-axis pebble imbrication data from the
Hemis Conglomerate Formation, Zanskar Gorge. (e) Flute and
groove casts from the Nimu Formation, Zanskar Gorge. (f) Flute
and groove casts from Nimu Formation, Saspol.
Choksti Conglomerate Formation. The Choksti Conglomerate
Formation is exposed immediately south of Choksti village
along the Zanskar Gorge (Fig. 4a). It is at least 300 m thick,
with the lower 100 m of the succession being dominated by
medium to thin-bedded laterally variable, fine to medium
grained sandstones. Palaeocurrents were predominantly to the
NE to east (Fig. 7c). The upper 200 m of the formation is
dominated by amalgamated, thick-bedded (up to 8 m) conglomerates (Fig. 8). The conglomerates are dominantly, clastsupported with a very poorly sorted, very coarse sandstone
matrix. Sandstone beds within the conglomerates are locally
planar cross-bedded. Larger clasts (up to 30 cm) are rounded,
with smaller clasts being more angular. Clast types higher in
the succession are dominated by red and green metasediments
with approximately 10% volcanics and 10% granodiorite. The
granodiorite content can get up to 60% lower in the succession.
The overall succession is divided by two red mudstone units,
approximately 4 m thick.
Conglomerate deposition occurred during ephemeral, high
bedload and suspended load transport resulting in a-axis
imbrication of pebbles; a fabric most typically found in
deep-water conglomerates (Harms et al. 1982). However, the
intercalated red mudstones suggest deposition in a continental
environment; an ephemeral, sheetflood dominated alluvial fan
(Blair & McPherson 1994), or a braided fluvial system are
thought the most likely candidates for the environment of
deposition.
INDUS GROUP SEDIMENTATION
Fig. 8. Sedimentary section through the Choksti Conglomerate
measured from the Zanskar Gorge, opposite Choksti village.
Hemis Conglomerate Formation (late Eocene/early Oligocene;
Van Haver 1984). The Hemis Conglomerate Formation is best
developed in the east of the study area (Van Haver 1984;
Searle et al. 1990), and were recorded in this study north of
Martselang (Fig. 4c) and north of Hemis. Their maximum
thickness is approximately 1500 m in the region of Hemis, with
only a thin remnant (150 m) preserved 50 km west in the
Zanskar Gorge. This formation is characterized by laterally
continuous (over several kilometres), 20–50 m thick conglomerate and sandstone units interbedded with red mudstones. The conglomerates are well-bedded, with a clast
supported texture, commonly imbricated, with well rounded
clasts up to 30 cm. Clast lithologies are dominated (up to 90%)
by granodiorites, acidic lavas and tuffs which are thought to
have been derived from the Ladakh Batholith (Van Haver
1984), along with a few metasediments. Locally interbedded
coarse sandstones show 75 cm thick, trough cross-sets. Overall,
the conglomeratic units show a slight fining and thinning up,
and their bases range from abrupt to gradual. The intervening
red mudstones can locally develop up to 40 m thick, and
contain thin (1–5 cm), laminated, fining-upward siltstone beds,
with some burrowing and wood fragments scattered along
bedding planes. Locally developed are green, branching networks believed to represent rootlets. No palaeocurrent
measurements were obtained from this formation. This succession is interpreted as the intercalation of high-energy (possibly
ephemeral) fluvial channels and floodplain mudstones in the
east of the area, thinning westward.
In the westernmost exposures of these conglomerates
located 1.5 km south of the confluence of the Indus and
Zanskar rivers (Figs 4a & 9), the conglomerates have a
different sedimentary character, and have previously been
157
Fig. 9. (a) Sedimentary section through the Hemis Conglomerate
and Nimu formations measured along the Zanskar Gorge.
(b) Close-up of sandstone/mudstone alternations from the Nimu
Formation interpreted as a succession of turbidites. Palaeocurrents
measured from cross-bedding are shown by the arrows with arrowed
bases, data from flutes shown with dots at base; these data are
plotted in Fig. 7.
interpreted as the thrusted repetition of the Choksti Conglomerate Formation (Searle et al. 1990). However, their
stratigraphic position at the base of the Nimu Formation, and
their granodiorite-clast-rich nature links them more closely
to the Hemis Conglomerate Formation, and here, they are
interpreted as along strike equivalents.
Near the base of the conglomerates in the Zanskar Gorge is
a 20 m thick, clast-supported conglomerate dominated by
granodioritic boulders and mudstone clasts (Fig. 9). The base
of this conglomerate cuts down at least 10 m into the underlying coarse sandstones. The upper part of the conglomerate
unit fines upward into very poorly sorted, coarse sandstones
with isolated, scattered boulders. The upper part of the formation comprises 1–10 m thick, amalgamated, medium to coarse
sandstones rich in mudstone clasts up to 3 m across. Beds are
normal to non-graded, and are laterally variable in thickness,
and are sharp based, with extensive downcutting. Cross bedding is rarely developed, with cross-sets up to 20 cm thick.
Locally developed are mudstone beds 0.1–4 m thick. Palaeocurrent data indicate a predominantly north to northnortheastwards directed flow, with a minor southwestward
component (Fig. 7). This succession is conformably overlain by dark grey mudstones and sandstones of the Nimu
Formation.
This part of the Hemis Conglomerate Formation is interpreted as the deposits of amalgamated, very coarse, highconcentration turbidity currents with a conglomeratic bedload
(Lowe 1982). The extensive downcutting, and abundance of
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H. D. SINCLAIR & N. JAFFEY
mudstone clasts and rafts suggests that these beds accumulated
in channels cutting into a mud-rich, northward dipping,
sub-aqueous slope. It is thought that these deposits record
sedimentation on a northward facing, fan-delta slope shed
from coarse fluvial systems similar to those recorded in the
Hemis Conglomerate Formation; examples of fan delta-fed
turbidite systems are summarized by Massari & Colella (1988),
classifying them as slope-type fan-deltas.
Nimu Formation. The Nimu Formation forms the first
outcrops found on the southern bank of the modern Indus
River between the village of Saspol in the west, eastward to
Martselang (Fig. 3). In this study, the Nimu Formation has
been studied from east to west at Stok, Pic Nimu, the Zanskar
Gorge and Saspol (Fig. 3). The maximum observed thickness
of the Nimu Formation was 600 m at Pic Nimu, although it is
likely that the total stratigraphic thickness is considerably
greater.
Near Stok Palace, Stok, the Nimu Formation comprises
0.04–1.5 m thick, coarse to fine sandstones intercalated with
red mudstones and thin siltstones. The sandstones show variable grading, with planar, rippled and undulatory lamination.
Abrupt basal surfaces locally show bottom structures such as
grooves. The siltstones are intensely rippled, with bedding
planes showing original bedforms with straight crests, and
symmetrical and trochoidal cross-sectional geometries;
features typical of wave generated ripples (Allen 1982).
Imprints of small bivalves are common on bedding surfaces;
Van Haver (1984) identified them as a freshwater bivalve of the
genus Unio. Burrowing is common in the finer deposits of this
succession. At Pic Nimu, the facies are similar, but sandstone
beds are thicker (up to 2 m), and occur as sandstone dominated packages ranging from 100 to 200 m thick; similarly, the
intervening mudstones are up to 100 m thick. The succession at
Stok and Nimu suggest episodic sand accumulation in a
shallow, sub-aqueous environment; it has been suggested that
the system was deltaic (Van Haver 1984).
In the Zanskar Gorge and at Saspol, the Nimu Formation
comprises a different suite of facies. However, whether they are
time equivalent to the deposits of Pic Nimu and Stok (Searle
et al. 1990) is questionable. The structural cross-section from
the Zanskar Gorge suggests that the Nimu Formation south of
the confluence has been structurally overthrusted onto the
more northerly Nimu Formation (Fig. 4a) which is traceable
2 km east to Pic Nimu. It is therefore proposed that the
exposures at Pic Nimu are younger than those in the Zanskar
Gorge. In the Zanskar Gorge, the facies comprise sandstone/
mudstone alternations (Figs 9 & 10) with coarse sandstone
beds up to 2 m thick, most showing normal grading, sharp
bases and more gradually fining tops. Flutes and grooves are
common on the base of beds, indicating palaeo-flow to the
north (Figs 7e & 9b). Mudstone clasts up to 1 m across are
common in the thicker beds. Amalgamation of coarse sandstone beds forms sandstone packages up to 12 m thick
(Fig. 9b). Thinner-bedded medium sandstones are also sharpbased and normally graded, and contain extensive currentripple lamination, again indicating northward palaeo-flow.
The mudstones are dark grey, and finely laminated.
In the region of Saspol (Fig. 3), the facies are similar to those
in the Zanskar Gorge, but show an increase in sandstone
thickness (up to 4 m), with more pebbly, and partially erosive
bases. Planar cross-bedding is also more common. Palaeo-flow
from grooves and flutes in the Saspol region was directed
primarily to the SE (Fig. 7f).
Fig. 10. Field drawing of unconformity at top Nimu Formation,
overlain by undeformed Quaternary alluvial sediments. Location
approximately 500 m south of the confluence of the Zanskar and
Indus rivers, southwest of Pic Nimu (Fig. 3).
The Nimu Formation is deformed throughout the region.
Their upper surface is either the present-day erosion surface, or
a palaeo-erosion surface that is now blanketed by undeformed
sands and gravels (Fig. 10) deposited by alluvial fans, and
Quaternary lakes caused by the damming of the Indus valley
(Bürgisser et al. 1982).
Palaeo-environments of the Indus Group; evidence for
the palaeo-Indus
The intermontane basin (post-Nurla Formation) was infilled
by sediments deposited in environments which alternated
between fluvial and (probably) lacustrine (Fig. 11). The Nurla
Formation and Choksti Conglomerates record coarse-grained,
braided and meandering channels interbedded with floodplain
Fig. 11. Summary of sedimentary succession exposed along the
Zanskar Gorge. Columns on right-hand side indicate formation
names and approximate palaeo-environmental interpretations. The
zig-zagged lines through the section indicate the location of major
thrust faults.
INDUS GROUP SEDIMENTATION
mudstones. There is no evidence of major fluvial incision, and
there is high preservation of floodplain muds, suggesting that
the bulk of this succession is aggradational (Bridge & Leeder
1979). The palaeo-currents during this period were highly
variable, with a predominantly northeasterly flow in the
Choksti Conglomerate; this is highly at variance with the
WNW directed flow required to suggest these sediments record
the palaeo-Indus.
The Nimu Formation and western outcrops of the Hemis
Conglomerate record >300 m of sediment deposited dominantly by mass flows and turbidity currents, at water depths
below the influence of fluvial or wave activity, representing a
water depth of >20 m, and possibly significantly more. It has
been suggested that these formations were deposited in a large
lake (Van Haver 1984; Searle et al. 1990). Palaeocurrents in
these formations are again variable, with a northward
and south-eastward component (Fig. 7). The upper Nimu
Formation records a regression into shallow water environments. As with the previous formations, the Nimu Formation
does not appear to record the early development of the
Indus River. The Hemis Conglomerate records high energy
fluvial sedimentation. However, its local appearance and
westward termination preclude its interpretation as a
sustained, basin-parallel flowing fluvial system.
Overall, the Indus Group record sedimentation within
variable palaeo-environments. Palaeocurrents are oblique or
perpendicular to the modern structural strike, and to the
longitudinal axis of the palaeogeographically restored Indus
Basin (Van Haver 1984; Garzanti & Van Haver 1988; Garzanti
et al. 1996). The limited range in compositions found within
the clasts of the conglomerates (Van Haver 1984), and their
dominantly local origin (from the Ladakh magmatic arc),
favour sedimentation from internally drained, transverse
fluvial and fluvio-deltaic fan systems as suggested by the
palaeocurrent data.
None of the sedimentological data suggests that there was
any time period of sustained, fluvial discharge to the NNW
that might record sedimentation by a precursor fluvial system
to the modern Indus. Therefore, the Indus River is not thought
to have initiated along the Indus suture zone until after
deposition of the Indus Group.
Burial and exhumation of the Indus Group
Maximum burial temperatures for the Indus Group can be
estimated from illite crystallinity values presented by Van
Haver (1984); values measured range from an illite crystallinity
index of 4 in the Nummulitic Limestones to 9.8 in the Nurla
Formation. These values are inversely proportional to depth
(Zen & Thompson 1974), and hence the maximum burial
depth is recorded by the Nummulitic Limestones. Correlations
of illite crystallinity values with coal rank and vitrinite
reflectance values (Zen & Thompson 1974; Cornford 1984)
allow estimates of the maximum possible temperatures
reached by the Nummulitic Limestones of 280C. Maximum
possible temperatures reached by the Nurla Formation (illite
crystallinity index=10) are 155C. The difference between these
two values (125C) translates to a differential burial of 2.5–
5 km assuming a possible range of geothermal gradients from
25–50C km1. This would allow for the estimated total
thickness for the succession of 3.5 km.
Apatite fission track thermochronology enables an approximation of the time at which an apatite crystal passed through
159
Fig. 12. Time temperature plot for the Indus Group and for the
Ladakh Batholith. Illite crystallinity data give estimates of maximum
temperatures experienced by the Indus Group. Since the values are
greater than the temperature of the apatite fission track partial
annealing zone (120–60C), the apatite fission track ages will have
been reset from the source rock age during burial. The neighbouring
Ladakh Batholith has older apatite fission track ages indicating
slower cooling rates during exhumation. The more rapid cooling of
the Indus Group is interpreted as a result of structurally induced
uplift and erosion during Zanskar backthrusting.
the 110C (10C) isotherm during cooling to the surface
(Brown et al. 1994). By combining the age at which a unit was
at 11010C with its maximum burial temperature, and
assuming a linear cooling path, it is possible to estimate the
time at which maximum burial took place. This is used to
approximate the timing of transition from sediment accumulation (burial) to erosionally driven exhumation within the
Indus Basin.
Apatite fission track ages from the Indus Group of the
Ladakh area are currently limited to two samples collected by
P. Clift, and processed at University College London by A.
Carter (Clift, unpublished data). One sample from the Nimu
Formation near the village of Nimu which has a central age of
143 Ma, and one from a boulder in the Choksti Conglomerate from the Zanskar gorge with a central age of 12 Ma.
Plotting these values on to a temperature/time plot (Fig. 12),
and projecting the line of cooling up to the maximum burial
temperature for the Nurla Formation enables a broad time
interval of 26–14 Ma in which cessation of sedimentation and
initiation of exhumation in the basin began (Fig. 12). This
correlates well with the youngest preserved sediments found to
the west in the Kargil area which are early Miocene (16–
25 Ma). Apatite fission track ages from the neighbouring
Ladakh Batholith immediately north of the Indus River
(Fig. 3) record older dates than those found in the Indus
Group. Two central ages for the batholith at Upshi are 283
and 292 Ma.
This information on the burial and exhumation of the Indus
Group suggests that sediment accumulation in the Indus Basin
terminated in early Miocene times. The transition from fluvial
aggradation to subsequent fluvial downcutting records baselevel lowering most likely associated with structurally driven
surface uplift (Ouchi 1985). Long-term fluvial incision is the
driving force behind erosional denudation and the consequent
exhumation of rock to the surface (Anderson 1994; Burbank
160
H. D. SINCLAIR & N. JAFFEY
et al. 1996). The average rate of cooling can be translated into
mean incision rates assuming a geothermal gradient. Early
Miocene geothermal gradients of 40C km1 have been calculated in Kohistan to the west of this area using fission track
data (Zeitler 1985); in this calculation a range of 30 to 50C
km1 is used. The apatite fission track ages from the Indus
Group translate to cooling rates of 5 to 11C Ma1 which
equate to long-term incision/denudation rates of 0.1 to 0.4 mm
a1. These rates are slow compared to values from the Indus
valley around the Nanga Parbat massif of up to 10 mm a1
based on apatite fission track ages (Burbank et al. 1996).
The cooling rates from the neighbouring Ladakh Batholith
were slower, ranging from 3.2 to 4.8C Ma1 which equate to
denudation rates of 0.06–0.16 mm a1. The contrast in denudation rates from the lower values in the Ladakh Batholith to
the higher values in the Indus Group is interpreted to result
from the northward thrusting of the Zanskar and Indus
sedimentary rocks against the undeformed buttress of the
batholith, i.e., structurally driven surface uplift and erosion.
Initiation of the palaeo-Indus and regional uplift
The data presented comprise a sedimentary succession which
records accumulation in an internally drained intermontane
basin (Fig. 13a) rather than evidence of a major westwardflowing river system from Eocene to early Miocene times, and
a cooling history which reveals relatively slow denudation in
the area since then. Hence, it is proposed that the earliest
possible time for initiation of a palaeo-Indus river in this area
was early Miocene (i.e. <26 Ma) as opposed to 45–55 Ma
suggested by Searle (1996), Qayyum et al. (1997) and Clift
et al. (2000).
The termination of sedimentation along the Indus suture
zone, and the onset of denudation in early Miocene times
relates to the larger scale development of the Himalayas at that
time. The Indus valley is bordered to the SW by the Zanskar
range and High Himalayas. Maximum P–T conditions, indicative of maximum crustal thickening in these areas, occurred
during Late Oligocene to Early Miocene times (Searle et al.
1992). Comparisons of detrital zircon ages with their stratigraphic ages from molasse sedimentary rocks indicate regional
accelerated unroofing of the Himalayas commencing in early
Miocene times (Harrison et al. 1993), associated with initiation
of the Main Central Thrust (Harrison et al. 1998). Similar
results have been interpreted from sandstone provenance
studies on the Murree and Siwalik supergroups recording the
infill of the foreland basin (Parkash et al. 1980; Critelli &
Garzanti 1994).
The combination of maximum burial ages, and accelerated
unroofing of the Himalayas during early Miocene times suggests that this was the time at which the main Himalayan chain
was initiated as a zone of high elevation and relief (Searle
1995). This period of thickening and uplift is likely to have
been the mechanical force necessary to initiate northwarddirected (back-) thrusting of the Zanskar and Indus sedimentary rocks resulting in termination of sedimentation in the
Indus intermontane basin, and its subsequent erosional
unroofing (Fig. 13b). The differential uplift field as recorded
in the cooling histories between the backthrust Zanskar
and Indus sediments and the relatively undeformed Ladakh
Batholith provides a mechanism for the development of a
major longitudinal river system such as the Indus (Koons
1995).
Fig. 13. Approximate palaeogeographic reconstructions along the
Indus Suture Zone during Oligocene and Miocene times. (a) During
Oligocene times, the elevation of the present Tibetan Plateau was
between 1 and 3 km, with the incipient Himalayan mountain belt
remained relatively low (Harrison et al. 1998). Additional
piggy-back basins were located over this area, but are not shown at
this scale (Critelli & Garzanti 1994). Sedimentation of the Indus
Group along the Indus Suture Zone fluctuated between fluvial,
deltaic and deeper water environments, with variable palaeocurrent
directions, and a predominantly local source (Van Haver 1984);
these sediments do not represent sedimentation by a
north-northwestwardly through-flowing river that could be
interpreted as a palaeo-Indus River. (b). Accelerated regional uplift
in Early Miocene times resulted in the termination of sedimentation
along the Indus Suture Zone, and the onset of fluvial incision
driving denudation. Differential uplift between the actively
backthrusting Zanskar and Indus sedimentary rocks and the
undeformed Ladakh Batholith provided a mechanism for the
initiation of a longitudinal river system. At a similar time, the Indus
Delta and submarine fan prograded rapidly into its present position.
It is possible that this may have been linked, and that it records the
timing of initiation of the main Indus River as a longitudinal
drainage network, but this remains speculative.
Evolution of the Indus River system
The earliest evidence for some form of westward-flowing
drainage network in the western Himalayas comes from
the late Eocene to early Miocene Khojak Formation in the
INDUS GROUP SEDIMENTATION
Katawaz Basin of Pakistan (Qayyum et al. 1997, in press).
These sedimentary rocks record a delta and submarine fan
complex that accumulated in a remnant ocean basin on the
western margin of the evolving collisional belt. Qayyum et al.
(in press) provide sandstone provenance data that indicate that
the Khojak Formation was derived from a collisional orogen,
and not a magmatic arc. These data make it highly unlikely
that the Indus Group, which is rich in material derived from
the Ladakh Batholith, could represent a proximal feeder
system for the Khojak Formation.
The present-day Indus floodplain of eastern Pakistan was a
shallow seaway called the Kirthar Gulf in Oligocene to Middle
Miocene times (Kazmi 1984). The northern margin of the
Kirthar Gulf is recorded by the Bugti bone beds (Pascoe 1964),
which contain a transition from coastal to marine deltaic
environments in Early Burdigalian times, recording the initiation of southward progradation of the Indus Delta (Kazmi
1984). Accelerated sedimentation in the present position of the
Indus submarine fan appears to have commenced in Late
Oligocene/Early Miocene times (Whitmarsh et al. 1974; Kolla
& Coumes 1987; Davies et al. 1995), although much of the
deep-sea drilling information is on the periphery of the main
Indus Fan, which is likely to have been initiated later than
more proximal parts.
The earliest evidence for a palaeo-Indus river of comparable
tectono-geomorphic status to the present-day river comes from
foreland basin deposits of the Siwalik Group (Abassi & Friend
2000). The Janak Conglomerate on the western margins of the
Potwar Plateau was deposited from 9 to 1 Ma, and records
sedimentation from a large braided river sourced in the
High Himalaya of northern Pakistan, and interpreted as the
palaeo-Indus (Abassi & Friend 2000).
It is evident that the southward progradation of the Indus
Delta and initial development of the Indus submarine fan
started by at least end Oligocene/beginning Miocene times.
Whilst there is evidence of sedimentation in an intermontane
basin (Indus Group) and a remnant ocean basin (Khojak
Formation) prior to this, it would be incorrect to describe these
as the deposits of a palaeo-Indus River. These palaeogeographic considerations support the notion that accelerated
unroofing of the Himalayas at end Oligocene/beginning
Miocene times was associated with the expansion of a paleoIndus delta/submarine fan system. At the same time, differential uplift between the Zanskar and Indus sedimentary rocks
and the Ladakh Batholith provided a mechanism for the
initiation of a major longitudinal river system along the route
of the modern Indus River (Fig. 13b). Whether the embryonic
Indus delta was initiated by a river following this route is
difficult to assess, however, the earliest sedimentological record
for a palaeo-Indus river of comparable tectono-geomorphic
status to the modern system starts at 9 Ma.
Conclusions
(1) The Indus Group of Ladakh, northwestern India, records
sedimentation from early Eocene times in an intermontane
basin. Sedimentation occurred in fluvial, shallow and deepwater environments. Palaeocurrent data through the Indus
Group are variable, recording flow to the south, SE, north
and NNE. The environmental reconstructions, sediment
provenance and palaeocurrent data suggest that the Indus
intermontane basin was internally drained, and does not
represent sedimentation by a through-flowing palaeo-Indus
River shedding material west-northwestward.
161
(2) Maximum burial temperatures for the Indus Group
using illite crystallinity ranged from 155 to 280 C. Apatite
fission track ages for the Indus Group range from 12 to 14 Ma.
Combining these data with stratigraphic information from
the along strike equivalent sediments of the Kargil area
suggests that the Indus Basin accumulated sediment until early
Miocene times, when erosional unroofing was initiated. Timeaveraged rates of unroofing since then have ranged from 0.1 to
0.4 mm a 1; this is relatively fast compared to the unroofing
of the neighbouring Ladakh Batholith which ranged from
0.06–0.16 mm a 1.
(3) The early Miocene was a time of accelerated uplift
throughout the western Himalaya, and is coincident with the
time of initiation of the Indus Delta/submarine fan system. It is
proposed that this is the earliest time at which a palaeo-Indus
River of similar tectono-geomorphic status to the present
system could have been initiated. Differential uplift between
the Zanskar and Indus sedimentary rocks and the Ladakh
Batholith provide a mechanism for the initiation of a longitudinal river at this time following the route of the modern
Indus. Although, whether this was the location of the main
trunk stream feeding the embryonic Indus delta remains
speculative.
M. P. Searle first introduced us to the area and helped enormously
with logistics and discussions of the geological setting for which are
extremely grateful. T. Crosby, V. Sinclair and F. Hussein are thanked
for assistance in the field. P. Clift is thanked for the provision of
unpublished data. The paper was written whilst on sabbatical at
Pennsylvania State University, where much logistical support and
scientific stimulus was given by D. W. Burbank and colleagues. H.D.S.
was partly funded by a Birmingham University Science Faculty pilot
research scheme. Reviews by E. Garzanti and P. Friend, plus editorial
comments by D. Pirrie are much appreciated. M. Qayyum is thanked
for providing pre-prints of forthcoming publications. Acknowledgement of the above colleagues does not necessarily mean they all agree
with the interpretations presented.
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Received 6 August 1999; revised typescript accepted 26 July 2000.
Scientific editing by Duncan Pirrie.