International Geology Review, Vol. 49, 2007, p. 798–810.
Copyright © 2007 by V. H. Winston & Son, Inc. All rights reserved.
Early Orogenic History of the Eastern Himalayas:
Compositional Studies of Paleogene Sandstones from Assam,
Northeast India
ASHRAF UDDIN,1 PRANAV KUMAR,
Himalayan Research Laboratory, Department of Geology and Geography, Auburn University, Auburn, Alabama 36849
AND
J. N. SARMA
Department of Applied Geology, Dibrugarh University, Dibrugarh 786004, Assam, India
Abstract
Thick Eocene–Oligocene sequences, exposed near the Margherita-Changlang area, northeast
Assam represent detritus derived from the early Himalayan and Indo-Burman orogenic belts,
extending the 18–0 Ma record recovered from drilling the distal Bengal Fan. Sandstones from the
Eocene Disang Group (Qt68F3L29; total quartz–feldspar–lithic fragments) and the lower Oligocene
Naogaon Formation (Qt69F6L25) are compositionally and texturally immature, composed mainly of
quartz, sedimentary and low-grade- metamorphic lithic fragments (including abundant chert), and
plagioclase. Sandstones of the overlying middle and upper Oligocene Baragolai (Qt66F12L22) and
Tikak Parbat (Qt82F4L14) formations are similar but also contain significant amounts of volcanic and
higher grade metamorphic detritus. These sandstones are clearly derived from an orogenic source,
exposing and eroding sedimentary and low-grade metamorphic units to form the older sandstones,
followed by increasing contributions from volcanic and higher grade metamorphic rocks during deposition of the middle and upper Oligocene sandstones. In contrast, Eo-Oligocene strata (Eocene:
Qt99F1L0; Oligocene: Qt90F3L7) from the neighboring Bengal Basin contain angular quartzose sands
that represent first-cycle detritus, most likely from the Indian craton. The Bengal Basin was protected from orogenic sedimentation during the Eocene–Oligocene, either by a barrier to sediment
transport (a peripheral forebulge or a marine basin) or by distance, prior to the approach of the basin
toward Asia. Motion of this part of the Indian plate relative to now-adjacent Southeast Asia was most
likely accomplished along strike-slip faults, like the N-S–trending Kaladan fault, located just east
of the Bengal Basin. Similarity in modal composition (quartzolithic to phyllarenitic) of Paleogene
sequences of Assam and basins south of the Himalayan western syntaxis suggests that the Himalayan emergence was not strongly diachronous, with initial collision and uplift at both syntaxial
areas occurred in the Eocene.
Introduction
THE COLLISION OF India with Eurasia provides a
spectacular lesson in plate tectonics. Timing of the
collision near the eastern syntaxis is very poorly
known (Packham, l996; Rowley, l996), however, and
improved resolution on the timing would aid in
developing more accurate models for deformation in
the eastern Himalayas. Data bearing on the timing
of collision come mainly from areas west of the central Himalayas. Although most workers suggest that
India began to collide with Eurasia at around 50 Ma,
others propose an earlier collision at about 70 Ma
1Corresponding
author; email: uddinas@auburn.edu
0020-6814/07/950/798-13 $25.00
(Yin and Harrison, 2000). Even less well understood
is the location of the boundary between India and
Indochina through time. Most workers place the
main boundary between India and Indochina for the
past 13 million years along the Sagaing fault in
Myanmar (formerly Burma; Mitchell, 1993; Fig. 1).
Total displacement on the Sagaing fault is not well
known, but evidence on offset of ophiolitic rocks,
and on opening of the Andaman Sea suggest about
400 to 500 km of right slip (Curray, 1989). NUVEL1A plate reconstructions place Assam, the northeast
corner of India, some 3000 ± 250 km south of
Eurasia at about 50 Ma, and more recent reconstructions decrease this by only a few hundred kilometers (Gordon et al., 1999).
798
PALEOGENE SANDSTONES FROM ASSAM, NORTHEAST INDIA
799
FIG. 1. Map of South Asia showing lithotectonic belts of Himalayan and Indo-Burman orogens and locations of
Assam and the Bengal Basin, along with other reference locations mentioned in the text. The Indian shield and Shillong
Plateau expose Precambrian crystalline rocks. Approximate limits of the Indus and Bengal fan are shown. Deep Sea
Drilling Project sites 217, 218, 222, 223, and 224 and area drilled by Ocean Drilling Program Leg 116 are shown in the
Bengal and Indus fans. Framed area is shown in detail in Figure 2 (after Uddin and Lundberg, 1998a).
Evidence of the early collision in the eastern Himalayas should be recorded in the stratigraphic record
of basins south of the mountain belt. Paleogene
strata of the deep-sea Bengal fan have not yet been
recovered (only back to about 18 or 17 Ma; Cochran,
1990). Paleogene sandstones of the onshore delta of
the Bengal Basin are quartzose, suggesting derivation most possibly from non-orogenic sources
(Uddin and Lundberg, 1998a). More proximal to the
eastern Himalayas is the Assam Basin of India, a
foreland basin with over 6 km of Eocene to Pleistocene marine to terrestrial strata deposited on
continental crust. Thus it is anticipated that the
initiation of collision may be recorded by these
predominantly non-marine or deltaic strata, in that
collision likely began in the submarine realm. However, considering the modern Taiwan collision, it
appears that a sizeable mountain belt can emerge in
a relatively short time span (within 1 m.y.; Dorsey,
1988). In the case of Taiwan, shallow-marine to nonmarine sediments were deposited on continental
crust of the downgoing plate within 1 m.y. or so from
the inception of collision (Covey, 1986). Furthermore, it is important to note that the Assam
sequence records the very early collision, because
the initial detritus is rich in sedimentary lithic fragments and it subsequently shifted to dominance by
meta-sedimentary lithic fragments.
This study reports modal analyses of Eocene and
Oligocene sandstones exposed near the MargheritaChanglang area of northeast Assam, India. Compositional data were collected to constrain the provenance of these deposits, and to compare them with
coeval sequences elsewhere in the foreland; this
should help decipher the early erosional record of
the eastern Himalayas in order to further elucidate
800
UDDIN ET AL.
FIG. 2. Map showing the Assam and Bengal basins and their tectonic elements such as the eastern Himalayas and
Indo-Burman Ranges. Areas enclosed by the Naga and Disang thrusts form the Schuppen belt. Samples for this study
were collected from the northeastern part of the Schuppen belt (Margherita-Changlang) of Assam. The Shillong Plateau,
Mikir Hills, and Mishmi Hills are uplifted blocks of Precambrian massifs. The Dauki fault demarcates the Shillong
Plateau from the Sylhet trough of the Bengal Basin. The Kaladan fault, located east of the Chittagong Hills of the Bengal
Basin, separates the Assam sequences from the Bengal Basin (after Hutchison, 1989).
the history of collision between northeast India and
Asia.
Regional Geologic Setting
The Assam Basin is bounded by the Indian craton and the Shillong Plateau, a Precambrian massif,
to the west; by the eastern segment of the Himalayas
to the north; the Mishmi Hills in the northeast; the
Indo-Burman Ranges to the east and immediate
south; and the Bengal Basin of Bangladesh and the
Bengal deep-sea fan to the southwest (Fig. 1). The
eastern Himalayan syntaxis is located only about
150 km NNE of Assam; parts of the Himalayas and
the Indo-Burman Ranges are located even closer
(Fig. 2). The northernmost extension of the IndoBurman Ranges merges with the E-W–trending
Himalayas at the Eastern Himalayan syntaxis. The
Himalayas consist of six longitudinal lithotectonic
units juxtaposed along generally N-dipping thrust
faults (Le Fort, 1996). From north to south (Fig. 1),
these are the: (1) Trans-Himalayas, consisting of
calc-alkaline plutons; (2) Indus suture zone, exposing ophiolitic bands representing the zone of collision between India and Eurasia; (3) Tibetan
Himalayas, represented by fossiliferous Cambrian
to Eocene sediments; (4) Higher Himalayas, located
north of the Main Central Thrust, composed of
schists, gneisses, and leucogranites; (5) Lower or
Lesser Himalayas, composed of unfossiliferous Precambrian and Palaeozoic sedimentary rocks, and
crystalline rocks; and (6) Sub-Himalayas, representing Miocene to Pleistocene molasse-type deposits of
the Siwaliks. The N-S–trending Indo-Burman
Ranges east and south of the Assam-Bengal system
consist of early Tertiary synorogenic sediments,
PALEOGENE SANDSTONES FROM ASSAM, NORTHEAST INDIA
schists, and ophiolitic belts (Fig. 1; Brunnschweiler,
1966; Sengupta et al., 1990). Crystalline rocks,
predominantly gneisses of Precambrian age, make
up the bulk of the Indian craton that is sporadically
overlain by Permian Gondwana deposits and Cretaceous flood basalts of the Rajmahal Traps (Hutchison, 1989). Crustal material of a pre-Gondwana
landmass crops out in the Mikir Hills, the Shillong
Plateau, and the Mishmi Hills, most of which lie
outside Assam. The Shillong Plateau, which is a
major geomorphic feature in the region, was uplifted
to its present height in the Pliocene (Johnson and
Nur Alam, 1991). The southern edge of the plateau
is bounded by the Dauki fault (Fig. 2; Uddin and
Lundberg, 2004).
Several thrust faults bound the MargheritaChanglang area of northeast Assam, including the
Naga thrust to the northwest and Disang thrust to the
southeast (Fig. 2). This thrust-bounded area is also
called the “Schuppen belt” (Rangarao, 1983). The
Naga thrust is a major décollement in the study area.
Thrusting began in the late Eocene or early
Oligocene and continued into the late Pliocene; total
shortening is estimated to be about 300 km (Evans,
1964; Saikia, 1999). The imbricate belt of the Naga
thrust developed through compression during subduction (Fig. 2; Saikia, 1999). Geomorphically, the
Assam and Bengal basins are separated by the Mikir
Hills, the Shillong Plateau, and the Schuppen belt.
Thick successions of Cenozoic basin fill have
been drilled and exposed in the Sylhet trough of the
Bengal Basin and uplifted along the Chittagong fold
belts of the eastern Bengal Basin (Fig. 2). The Chittagong fold belts comprise tight NNW-trending folds
along the eastern edge of the foredeep. The KohimaPatkai synclinorium is developed in the southern
and southeastern parts of the Schuppen belt, and
extends to the folded belt of the Sylhet trough and
Chittagong Hills (Fig. 2; Dasgupta, 1984). These
fold belts represent a series of N-S–trending anticlinal ridges and synclinal valleys, an arcuate belt
that is convex toward the west. The fold belt shows
an increase in structural complexity toward the east,
into the Arakan Yoma–Chin Hills and the IndoBurman Ranges (Fig. 2). The latter are bounded by
two N-S–trending right lateral faults, Sagaing to the
east and Kaladan to the west, adjacent to the Bengal
Basin (Uddin and Lundberg, 2004). Although the
Sagaing fault is commonly recognized as a rightlateral fault in Southeast Asia (e.g., Curray, 1989;
Mitchell, 1993; Uddin and Lundberg, 1998a), the
Kaladan fault is not that popularly known. Although
801
this has a thrust component (Sikder, 1998), designation of the Kaladan fault as a right-lateral one has
been promoted by Murphy (1988) and Zutshi
(1993). The Kaladan fault trends NE-SW along the
Kaladan River, between the eastern boundary of
Bangladesh and western Myanmar (Fig. 2; Murphy,
1988; Zutshi, 1993; Sikdar, 1998). This fault is
traceable on satellite images from the Disang thrust
on the north to offshore Myanmar on the south, a
distance of few hundred kilometers.
Assam Paleogene Sequences
The stratigraphic framework of Assam is based
mainly on biostratigraphy, predominantly using
palynology, with correlations depending on lithostratigraphy (Evans, 1964; Sinha and Sastri, 1973;
Rangarao, 1983). The basin sequences have also
been correlated by seismic stratigraphy by various
industry groups, including the Oil and Natural Gas
Commission of India (Saikia, 1999).
The Paleogene section of the Margherita-Changlang area used in this study (Table 1) comprises the
upper Eocene Disang Group (up to 3 km thick), the
lower Oligocene Naogaon Formation (up to 2.2 km),
the middle Oligocene Baragolai Formation (up to
3.3 km), and the upper Oligocene Tikak Parbat
Formation (~0.7 km; Table 1). The Oligocene formations make up the Barail Group. The thickness of
each unit decreases generally to the west (Rangarao,
1983).
The Disang Group is marine, based on marine
fossils, radiolarian cherts, and other typical deepmarine deposits. The Disang sequence consists of
fissile, carbonaceous mudrocks with fine-grained
sandstone. Nagappa (1959) reported arenaceous foraminifera from the top part of Disang and suggested
a late Eocene age. Evans (1964) found Nummulites
from sandy shale of Disang and suggested a late
Eocene age. The upper part of the Disang represents
an argillaceous facies analogous to the Eocene
Sylhet and Kopili formations (shelf equivalents in
the Upper Assam Plains and Mikir hills; Rangarao,
1983).
The Naogaon Formation consists mostly of finegrained sandstones with subordinate siltstones,
claystones, and shales, showing flaser and lenticular
bedding. The middle unit of the Baragolai Formation is dominantly argillaceous with thin siltstones
and sandstones. Shales in this unit are dark grey and
commonly show concretions. The Tikak Parbat Formation is composed dominantly of grey, moderately
802
UDDIN ET AL.
TABLE 1. Paleogene Stratigraphy of the Margherita-Changlang Area in Upper Assam
Chronostratigraphy
Oligocene
Late Eocene
Group
Barail
Disang
Formation
Tikak Parbat
Thickness (m)
Brief lithology
500 to 700
Sandstones, thin-bedded grey sandy siltstone
Baragolai
2700 to 3300
Predominantly shale with subordinate thin
sandstone beds and prominent coal seams
Naogaon
1040 to 2200
Thinly bedded sandstone, thin subordinate
shale
Disang
2000 to 3300
Fine-grained sandstone with subordinate
dark-gray shale rich in carbonaceous matter and massive siltstone with concretions
Source: After Sinha and Sastri, 1973 and Rangarao, 1983.
sorted sandstones; minor siltstones and thick coal
beds are also present in this unit. These Oligocene
units have been interpreted as brackish-water and
deltaic deposits (Rangarao, 1983).
Methods
Twenty-three representative Eocene–Oligocene
sandstone samples from Assam were selected for
modal analysis on the basis of appropriate grain size
and low alteration. Most of the samples are highly
indurated. A few unconsolidated sand samples
chosen were sieved, and the fractions coarser than
0.063 mm were epoxied into plugs for thin-section
preparation. Petrographic analyses were conducted
following the Gazzi-Dickinson method, counting
sand-sized minerals included in lithic fragments as
the mineral phases rather than the host lithic fragment (i.e., Ingersoll et al., 1984). All thin sections
were stained for plagioclase and potassium feldspar,
following techniques modified from Houghton
(1980). At least 300 framework points were counted
per sample, with 400 framework points counted for
samples with greater compositional diversity.
Selected thin sections were also counted a second
time by a different person in order to evaluate operator error.
Point-counting parameters and recalculated
parameters are defined in Table 2. Normalized
modal data are given in Table 3 and representative
photomicrographs are shown in Figure 3. Polygons
surrounding mean values are calculated as sample
standard deviations, although these do not represent
true standard deviations for constrained-sum data
(see Ingersoll et al., 1984); they are shown to indi-
cate the variability of values for each group. Ternary
diagrams using major detrital components, monocrystalline grains, and the phaneritic lithic fragments were constructed in order to visualize
variations in sand composition and to help interpret
the tectonic provenance (i.e., Dickinson, 1985).
Normalized modal data are depicted graphically in
Figure 4A and 4B.
Assam Paleogene Sandstone Compositions
Modal analytical data from Eocene–Oligocene
sequences in Assam are summarized below for the
various stratigraphic units, from oldest to youngest.
Disang Group
Sandstones from the Eocene Disang Group
(Qt68F3L29; Figs. 3A, 4A, and 4B) are composed of
fine- to medium-grained, subangular to angular
grains, containing mostly monocrystalline quartz,
and also foliated and equant polycrystalline quartz,
plagioclase, sedimentary and metamorphic lithic
fragments of phyllite grade and fine-grained quartzmica-chlorite schist. Sedimentary and low-grade
metasedimentary lithic fragments suggest derivation
of sediments from proximal orogenic sources. Like
the plagioclase, the large angular monocrystalline
quartz could have been derived from a volcanic
source, or possibly from a granitic source, although
the almost complete lack of alkali feldspar suggests
otherwise.
Naogaon Formation
Sandstones from the lower Oligocene Naogaon
Formation (Qt69F6L25; Figs. 3B, 4A, and 4B) are
PALEOGENE SANDSTONES FROM ASSAM, NORTHEAST INDIA
803
TABLE 2. Recalculated Modal Parameters of Sand and Sandstones Used in This Study
Quartzose grains (Qt = Qm + Qp), where
Qt = total quartzose grains
Qm = monocrystalline quartzose grains (> 0.625 mm)
Qp = polycrystalline quartz grains, including chert grains
Feldspar grains (F = P + K)
F = total feldspar grains
P = plagioclase feldspar grains
K = potassium feldspar grains
Unstable lithic fragments (L = Ls + Lv + Lm; L = Lsm + Lvm; Lt = Ls + Lv + Lm + Qp)
L = total aphanitic lithic fragments
Lt = total aphanitic lithic fragments, including polycrystalline quartz and chert
Ls = sedimentary lithic fragments, mostly argillites
Lv = volcanic lithic fragments
Lm = very low to intermediate grade metamorphic lithic fragments
Lsm = sedimentary and metasedimentary lithic fragments
Lvm = volcanic, hypabyssal, metavolcanic lithic fragments
Source: After Dickinson, 1985 and Uddin and Lundberg, 1998a.
quartzolithic and contain subangular to angular
grains of monocrystalline and polycrystalline quartz,
mostly plagioclase feldspar, and sedimentary and
metamorphic lithic fragments. Lithic fragments in
this unit are more diverse compared to the Eocene
Disang Group sandstones.
Baragolai Formation
Sandstones from the middle Oligocene Baragolai
Formation (Qt66F12L22; Figs. 3C, 4A, and 4B) comprise mono-and polycrystalline (also sheared) quartz,
feldspar (mostly plagioclase, with chlorite and epidote
inclusions), sedimentary lithic fragments of shale,
argillite and siltstone, and metamorphic lithic fragments of phyllite grade, fine- to medium-grained
quartzose-mica schists, and chlorite-quartz-epidotezoisite schists. Chert grains are abundant (Fig. 3C).
Volcanic lithic fragments are also present, mostly of
mafic lithologies with lathwork and local microlitic
textures. Some of these volcanic lithic fragments of
Baragolai sandstones show massive alteration to chlorite and possible epidote, probably representing a
mild metamorphic overprint, although some alteration
during burial diagenesis may also have occurred.
Lower and middle Oligocene sandstones also suggest
a proximal orogenic source because the detritus is
composed of sedimentary, metasedimentary, volcanic,
and metavolcanic lithologies. Sheared quartz grains
apparently were derived from zones of deformation. As
with the older Disang unit, the lack or near absence of
alkali feldspar suggests no significant granitic source
rocks for the lower Oligocene Naogaon Formation.
Lath-shaped plagioclase grains probably represent
volcanic phenocrysts. Chlorite-quartz-epidote (zoisite)
schists may have been derived from low-grade metamorphism of calcareous shales or mafic volcanic
rocks. Rare grains of amphibole also suggest a
medium-grade metamorphic source.
Tikak Parbat Formation
Sandstones from the upper Oligocene Tikak
Parbat Formation (Qt82F4L14; Figs. 3D, 4A, and 4B)
are texturally immature, with angular to subangular
fragments, and are coarser than the older units.
These upper Paleogene sandstones are also compositionally immature, consisting primarily of grains
containing monocrystalline quartz showing undulose
extinction. Sheared quartz, quartz-mica schist,
804
UDDIN ET AL.
TABLE 3. Normalized Modal Analyses of Paleogene Sandstones from Assam, India
QtFL (%)
F
QmFLt (%)
L
Qm
F
Lt
QmPK (%)
Sample number
Qt
Qm
P
T-17
95
1
4
59
1
40
99
1
T-12
78
8
13
42
8
49
83
11
K
QpLvmLsm (%)
LsLvLm (%)
Qp Lsm
Lvm
Ls
Lv Lm
0
90 8.9
0.6849
43
7
50
6
73
0
62
0
38
Tikak Parbat Fm. (Upper Oligocene)
27
T-10
83
4
14
49
4
47
93
4
3
71
29
0
63
0
37
T-7
77
3
19
58
3
39
94
3
3
50
50
0
56
0
44
T-1
74
5
21
33
5
62
86
14
0
66
31
3.0973
58
9
33
Mean (n = 5)
82
4
14
48
4
47
91
7
2
70
29
0.7565
56
3
40
Standard deviation
8.4
3
7
11
3
9
6.3
6
2
14
15
1.3418
8
5 6.8
Baragolai Fm. (Middle Oligocene)
B-25
70
17
13
37
17
46
68
8
23
71
29
0
53
0
47
B-21
64
8
28
48
8
44
85
12
3
37
63
0
26
0
74
B-19
62
17
21
49
17
34
74
24
2
38
53
8.6207
38
14
49
B-14
64
11
25
54
11
35
85
15
0
30
64
5.9322
45
8
47
B-7
72
6
22
54
6
39
89
11
0
45
40
14.754
28
27
45
B-4
67
10
23
54
10
36
85
15
0
37
55
8
39
13
48
Mean (n = 6)
66
12
22
49
12
39
81
14
5
43
51
6.2178
38
10
51
Standard deviation
3.8
5
5
6.6
5
5
8.1
5
9
15
14
5.6431
10
10
11
NB-2
73
3
24
0
22
77
1
15
1
83
Naogaon Fm. (Lower Oligocene)
66
3
31
96
4
NA-7
76
3
22
70
3
28
96
4
0
22
78
0
37
0
63
NB-6
72
10
18
52
10
39
84
10
5
54
46
0
58
0
42
NA-5
71
1
28
23
11
76
97
3
0
63
37
0
14
0
86
NA-2
52
13
35
47
13
40
78
3
18
12
84
4.6875
72
5
23
Mean (n = 5)
69
6
25
52
6
43
90
5
5
34
64
1.1375
39
1
60
Standard deviation
9.6
5
7
19
5
20
8.6
3
8
22
21
2.0312
25
2
27
D-23
74
4
22
51
4
45
93
7
0
51
49
0
68
0
32
D-22
64
3
34
50
3
48
95
5
0
30
70
0
73
0
27
D-16
78
1
22
61
1
39
99
1
0
44
56
0
89
0
11
Disang Group (Eocene)
D-15
67
2
30
56
2
42
96
4
0
27
69
4.0698
61
6
34
D-12
67
4
30
59
4
37
94
6
0
21
79
0
56
0
44
D-8
69
4
27
57
4
39
93
7
0
30
70
0
88
0
12
D-2
65
3
32
54
3
43
95
5
0
26
74
0
48
0
52
Mean (n = 7)
68
3
29
56
3
41
95
5
0
33
67
0.5814
69
1
30
Standard deviation
4.9
1
4
3.9
1
4
2
2
0
11
10
1.5382
16
2
15
Paleogene Mean (n = 23)
71
6
23
51
6
43
89
8
3
45
53
2.1733
51
4
45
Paleogene standard deviation
6.6
3
6
10
3
9
6.3
4
5
16
15
2.6386
8
4 8.7
PALEOGENE SANDSTONES FROM ASSAM, NORTHEAST INDIA
805
FIG. 3. Representative photomicrographs of sandstones from Assam, India. A. Eocene Disang Group: framework
grains are dominantly quartz (Qm), with sedimentary lithic fragments (Ls), plagioclase feldpars (plag), and chert grains.
B. Lower Oligocene Naogaon Formation: monocrystalline quartz grains (Qm), sedimentary lithic fragments (Ls), plagioclase feldspar (plag), and chert grains. C. Middle Oligocene Baragolai Formation: monocrystalline (Qm) and polycrystalline quartz grains (Qp), potassium feldspar (K-spar), and chert grains. D. Upper Oligocene Tikak Parbat Formation:
monocrystalline quartz grains (Qm), sedimentary (Ls) and metamorphic (Lm) lithic fragments, and mica. All these
framework grains suggest orogenic derivation. In contrast, the Oligocene Barail Formation from the Bengal Basin shows
subangular quartz grains with rare or no feldspar and lithic fragments (Fig. 6A; Uddin and Lundberg, 1998a).
chlorite-mica schist, black shale, and polycrystalline quartz (equant and foliated) are also present.
These sandstones also contain abundant stretched
quartz grains, chert, epidote, muscovite, and biotite.
Feldspars are sparse, and actinolite and epidote
schists are also rare. The presence of quartz-mica
schists, chlorite-mica schist, and the abundance of
detrital mica in the upper Oligocene to Neogene
sandstones suggest a low- to intermediate-grade
metamorphic source for the sandstones.
Interpretation of Assam Paleogene
Sandstone Modes
All Paleogene (Eocene and Oligocene) units
analyzed plot in the “recycled orogenic” provenance
fields of QtFL and QmFLt diagrams (Fig. 4A; Dick-
inson, 1985). These sandstones are quartzolithic
(Table 3; Q71F6L23) and phyllarenitic, and contain
more sedimentary and metasedimentary lithic fragments (Ls51Lv4Lm45). In the monocrystalline QmPK
diagram, most of the samples plot near the Qm pole
(Fig. 4A). Volcanic components are generally scarce
in Assam sandstones, with a peak in abundance in
the middle Oligocene Baragolai Formation that has
higher feldspar contents (Figs. 4A and 4B). Sample
T-17, which has a very quartzose composition, was
collected from strata that are probably transitional
between the Oligocene Tikak Parbat Formation and
the Neogene Surma Group (Table 3). For reference,
the Paleogene sandstones from the Bengal Basin are
also plotted in Figure 4 (Eocene—Be; Oligocene—
Bo). The sandstones from the Bengal Basin show
806
UDDIN ET AL.
FIG. 4. A. Ternary diagrams showing sandstone modes of Paleogene sandstones from Assam (QtFL, QmFLt, QmPK;
see Table 1 for definitions). Data plots show means (indicated by numbers 1–4 and standard deviation polygons for each
stratigraphic unit. Provenance fields are from Dickinson (1985). For comparison, distribution of the Paleogene Bengal
Basin samples (Be = Bengal Basin Eocene; Bo = Bengal Basin Oligocene) is also shown in all the diagrams (from Uddin
and Lundberg, 1998a). Note that the detrital modes of Paleogene sandstones from Assam plot in a “recycled orgenic”
field that is different from the the Paleogene sandstones of the Bengal Basin. Although standard deviations are not
strictly valid statistically for constant-sum, constrained compositional data, polygons are shown to indicate ranges of
values. B. Ternary diagrams showing lithic and polycrystalline modes of Paleogene sandstones from Assam (LsLvLm,
QpLvmLsm; see Table 1 for definitions). Data plots show means (indicated by numbers 1–4 and standard deviation
polygons for each stratigraphic unit. These plots do not show presence of much volcanic lithic fragments except the midOligocene Baragolai unit. For comparison, distribution of the Paleogene Bengal Basin samples (Be = Bengal Basin
Eocene; Bo = Bengal Basin Oligocene) is also shown in all the diagrams (from Uddin and Lundberg, 1998a). All these
plots show dominance of sedimentary and metamorphic lithic fragments in the Paleogene sequences of Assam. Volcanic
lithic fragments are not that significant, except in the mid-Oligocene Baragolai unit.
PALEOGENE SANDSTONES FROM ASSAM, NORTHEAST INDIA
more maturity (placed close to the quartz poles than
the Paleogene sandstones of Assam.
Paleogene Sandstones
across the Himalayan Foreland
Paleogene sandstones from Assam are compositionally quite different from coeval sandstones of the
adjacent deltaic Bengal Basin, but similar to coeval
sandstones of the foreland basins south of the western Himalayas. Eocene–Oligocene sandstone(s)
from the Bengal Basin are less indurated and are
dominantly quartzose (Qt90F3L7 to Qt99F1L0). Many
of the quartz grains are coarse and most are subangular to angular (Fig. 6A of Uddin and Lundberg,
1998a). Almost all are monocrystalline grains, with
very minor polycrystalline grains, and sedimentary
lithic fragments, scarce metamorphic lithic fragments, and no identifiable volcanic detritus. All of
the rare feldspar grains are potassium feldspars.
These quartz arenites are interpreted to have been
derived from the adjacent Indian craton (Uddin and
Lundberg, 1998a). The abundance of quartz and
scarcity of both feldspar grains and lithic fragments
in Bengal Basin sandstones also suggest a possible
source terrane with low relief, with intense chemical
weathering due to the position of the basin close to
the equator during the Paleogene. Given sufficiently
intense chemical weathering, the possibility also
exists that these quartzose sandstones were derived
from an orogenic source (Uddin and Lundberg,
1998a). In more close proximity toward the northwest of the Bengal Basin and west of Assam, but still
in the eastern half of the Himalayas, in the western
and central Nepal, the Paleocene fluvial to shallowmarine Amile Formation and Eocene marine to shallow-marine Bhainskati Formation are pure quartzarenites (Fig. 1; DeCelles et al., 1988). The lower
Miocene nonmarine Dumri Formation in western
Nepal is quartzolithic (Qt72F4L24; DeCelles et al.,
1998) with very little feldspar, most of which is
plagioclase. Zircon dates from these units suggest a
possible Himalayan source (DeCelles et al., 1998).
In the western Himalayan basins, the upper
Paleocene to lower Miocene synorogenic sediments
that began to fill the evolving foreland basins that
developed ahead of the southward-advancing Himalayas comprise terrestrial sediments of the lithofeldspathic Chulung La Formation (Fig. 1; Paleocene to
Oligocene; Qt24F26L50), quartzolithic tidal-flat to
fluviatile deposits of the Murree Supergroup (Paleocene to Oligocene; Qt68F5L27; Garzanti et al., 1987;
807
Critelli and Garzanti, 1994), the quartzolithic shallow-marine Subathu Formation (upper Paleocene to
middle Eocene; Qt63F7L30; Najman and Garzanti,
2000), and tidal flat to alluvial quartzolithic Dagshai
Formation (upper Oligocene; Qt58F1L31; Najman
and Garzanti, 2000; Fig. 1). Like the sandstones
from Assam and unlike the sandstones from the
adjacent Bengal Basin, these units contain abundant metasedimentary, volcanic, and sedimentary
lithic fragments and ophiolitic detritus, beginning
early in the Paleogene (Garzanti et al., 1987; Critelli
and Garzanti, 1994).
Early Orogenic History
of the Eastern Himalayas
The Eocene–Oligocene sandstones from Assam
were clearly derived from an orogenic source,
exposing and eroding sedimentary and low-grade
metamorphic units to form the older sandstones,
followed by increasing contributions from volcanic
and higher grade metamorphic rocks during deposition of the middle and upper Oligocene sandstones
(Fig. 5A). The Assam sandstones provide clear
evidence that orogeny had begun in the eastern
Himalayas by the Eocene, in contrast to the early
Miocene initiation suggested by the apparently firstcycle Paleogene quartz arenites (Uddin and Lundberg, 1998a, 1998b) and subsurface lithofacies
patterns of Miocene (Uddin and Lundberg, 1999) of
the Bengal Basin. The more proximal Assam
sequence apparently records the early stages of
orogenic activity; whereas the initial detritus is rich
in sedimentary lithic fragments, later sandstones
show a subsequent shift to dominance by metasedimentary lithic fragments.
Heavy-mineral contents in Oligocene sequences
from Assam are composed mostly of zircon, tourmaline, and rutile (ZTR) that are also associated among
others with chloritoid, epidote, garnet, hornblende,
kyanite, staurolite, and spinel, suggesting an orogenic source (Uddin et al., 2007). Microprobe study
of garnets and chrome-spinel grains from Paleogene
sequences of Assam also suggest a Himalayan
source material (or ophiolites) and/or the IndoBurmese ophiolitic belts (Kumar and Uddin, 2004).
Presence of dominantly ZTR minerals among the
nonopaque variety in the Eocene and Oligocene
sequences of the Bengal Basin suggests intense
post-depositional weathering and does not obviously
suggest an orogenic source (Uddin and Lundberg,
1998b). Heavy-mineral assemblages in both the
808
UDDIN ET AL.
FIG. 5. Schematic paleogeographic reconstruction of the
Himalayan and surrounding areas during the Paleogene time
showing tectonic elements of Assam, India, and Bengal Basin
in (A) pre-Miocene and (B) Miocene time. The Bengal Basin
may have been transported close to Assam during the
Miocene along right-lateral faults (i.e., the Kaladan fault)
located east of the basin.
Assam and Bengal basins become more diverse in
Miocene and younger formations, indicating derivation from orogenic belts (Uddin et al., 2007).
The Bengal Basin may have been protected from
orogenic sedimentation during Eocene and Oligocene time, either by a barrier to sediment transport
(a peripheral forebulge, or a marine basin, for example) or simply by distance (Fig. 5A). Early uplifts of
the Indo-Burman Ranges could potentially have
acted as a barrier; however, that seems unlikely
because the westward-encroaching ranges were
probably located farther east relative to the Bengal
Basin during the Paleogene than in the Miocene
(Mitchell, 1993; Uddin and Lundberg, 1999). These
compositional data also suggest that the Assam and
Bengal basins were latitudinally farther apart prior
to early Miocene time, and as a consequence, were
receiving detritus from two distinct sources. The two
sequences are presently exposed on either side
of the N-S–trending right-lateral Kaladan fault
(Murphy, 1988; Zutshi, 1993), between the eastern
fold belts of the Bengal Basin and western folds in
Assam, India (Figs. 1 and 2). This transpressional
fault seems to be resulting from oblique convergence of India with Indochina. India has been
moving both north and eastward; the northerly
motion has been attributed to the Miocene opening
of the Andaman Sea resulting in N-directed movement of India along right-lateral faults (Pivnik et al.,
1998). A strong candidate for such a fault is the
Kaladan fault (Uddin et al., 2007). These two distinct sequences were in close proximity by early
Miocene time because both are covered by lower
Miocene strata (the Bhuban Formation of the lower
Surma Group; Johnson and Nur Alam, 1991; Uddin
and Lundberg, 2004) that are similar in provenance
(Fig. 5B; Godwin et al., 2001; Uddin et al., 2007).
More regionally, the thick Eocene–Oligocene
sands from the Assam sequences are similar in composition to those of the western Himalayan foreland,
suggesting that the initial collision of Northeast and
Northwest India with Asia was not strongly diachronous. This non-diachronous convergence is also
supported by isotopic and compositional studies
(DeCelles et al., 1998) and paleomagnetic study
(Patzelt et al., 1996) and by work on subsequent
Miocene metamorphism and cooling history of the
two syntaxial areas (Nanga Parbat in the west and
Namche Barwa in the east; Ding et al., 2001). This
assumes, however, that Assam was initially part of
Indochina.
The active Kaladan fault appears to (geographically) separate the two Eocene–Oligocene
sequences in Assam and the Bengal Basin. Detritus
in the latter was apparently derived from the neighboring Indian craton, accumulating on crust of the
Indian plate prior to arrival of the clastic wedge
shed from the approaching orogeny. If true, then the
Miocene strata represent an overlap assemblage,
signifying the “docking” of this part of the Indian
plate with proximal terranes of Asia. One possible
PALEOGENE SANDSTONES FROM ASSAM, NORTHEAST INDIA
explanation of the contrast in sediment source is
that the part of the Indian plate represented by the
Bengal Basin was still far to the south of Asia until
the Miocene, when it arrived close enough to receive
detritus from the orogenic highlands fringing Asia’s
southern boundary. It is possible that the sequences
preserved in the Bengal Basin and Assam were originally deposited on two separate lithospheric plates,
with Assam as part of Indochina (Fig. 5A).
Conclusions
Paleogene sandstone composition from the study
area of northeastern Assam indicates recycled
orogenic derivation. The Assam sandstones differ
from coeval sandstones in the adjacent Bengal
Basin, which are texturally immature first-cycle
quartz arenites that were most likely derived from
the neighboring Indian craton. The Bengal Basin
was probably protected from orogenic sedimentation
during the Paleogene, either by a barrier to sediment transport or distance. If “distance” was the
cause, then the part of the Indian continent represented by the Bengal Basin was far to the south of
Asia until the early Miocene. Motion of this part of
the Indian plate relative to Southeast Asia (Indochina) was most likely accomplished along rightlateral faults, like the N-S–trending Kaladan fault,
located east of Bangladesh.
If the analyzed Paleogene sequences of Assam
were deposited on Indian continental crust, then the
Himalayan collision was not strongly diachronous,
with initial collision of both Northeast and Northwest India in the Eocene forming the two syntaxial
bends of the Himalayas. This suggestion of nondiachroneity is also supported by similarity in
composition and thickness of Paleogene strata in
basins (Assam, India and Pakistan) near the two
Himalayan syntaxes.
Acknowledgments
Thanks to Suvrat Kher for confirmational point
counts. Neil Lundberg, Clark Burchfiel, Roy Odom,
and Suvrat Kher helped with various aspects of the
work, including editing parts of the manuscript. Several students from Dibrugarh University (Assam)
helped during field work and sample collection for
the project. Mr. T. Bordoloi helped with logistics in
Digboi, Assam. Reviews by Peter DeCelles and Ray
Ingersoll have significantly improved the manuscript. M. Shamsudduha and Khandaker Zahid
809
helped draft some figures. PK received a grant-inaid support from Geological Society of America.
This manuscript is supported by U.S. National
Science Foundation grant EAR-0310306 awarded
to AU.
REFERENCES
Brunnschweiler, R. O., 1966, On the geology of the IndoBurman ranges (Arakan coast and Yoma, Chin Hills,
Naga Hills): Geological Society of Australia Bulletin,
v. 13, p. 137–194.
Cochran, J. R., 1990, Himalayan uplift, sea level, and the
record of Bengal fan sedimentation at the ODP Leg
116 sites, in Cochran, J. R., Stow, D. A. V., et al., eds.,
Proceedings of the Ocean Drilling Program, Scientific
Results, v. 116b: College Station, TX, Ocean Drilling
Program, p. 397–414.
Covey, M., 1986, The evolution of foreland basins to
steady state: evidence from the western Taiwan foreland basin, in: Allen, P. A., and Homewood, P., eds.,
Foreland basins: International Association of Sedimentologists Special Publication no. 8, p. 77–90.
Critelli, S., and Garzanti, E., 1994, Provenance of the
lower Tertiary Murree redbeds (Hazara-Kashmir syntaxis, Pakistan) and initial rising of the Himalayas:
Sedimentary Geology, v. 89, p. 265–284.
Curray, J. R., l989, The Sunda Arc: a model for oblique
plate convergence, in: Proceedings of the Snellius II
Symposium, theme: Geology and Geophysics of the
Banda Arc and adjacent areas, Part 1. Netherlands
Journal of Sea Research, v. 24, p. 131–140.
Dasgupta, S., 1984, Tectonic trends in Surma basin and
possible genesis of folded belt: Geological Survey of
India, Memoir, v. 113, p. 58–61.
DeCelles, P. G., Gehrels, G. E., Quade, J., and Ojha, T. P.,
1998, Eocene–early Miocene foreland basin development and the history of Himalayan thrusting, western
and central Nepal: Tectonics, v. 17, p. 741–765.
Dickinson, W. R., 1985, Interpreting provenance relations
from detrital modes of sandstones, in Zuffa, G. G., ed.,
Reading provenance from arenites: Dordecht, The
Netherlands, Riedel, p. 333–361.
Ding, L., Zhong, D., Yin, A., Kapp, P. and Harrison, T. M.,
2001, Cenozoic structural and metamorphic evolution
of the eastern Himalayan syntaxis (Namche Barwa):
Earth and Planetary Science Letters, v. 192, p. 423–
438.
Dorsey, R. J., 1988, Provenance evolution and unroofing
history of a modern arc-continent collision: Evidence
from petrography of Plio-Pleistocene sandstones, eastern Taiwan: Journal of Sedimentary Petrology, v. 58, p.
208–218.
Evans, P., 1964, The tectonic framework of Assam: Journal of the Geological Society of India, v. 5, p. 80–96.
810
UDDIN ET AL.
Garzanti, E., Baud, A., and Mascle, G., 1987, Sedimentary
record of the collision with Eurasia (Ladakh Himalaya,
India): Geodinimica Acta (Paris), v. 1, p. 297–312.
Godwin, T., Uddin, A., and Sarma, J. N., 2001, Provenance
history of Neogene sandstones from the Assam basin,
India [abs]: Geological Society of America Annual
Proceedings, Abstracts with Programs, v. 33, p. 72.
Gordon, R. G., Argus, D. F., and Heflin, M. B., 1999,
Revised estimate of angular velocity of India relative
to Eurasia: EOS (Transactions of the American
Geophysical Union), v. 80, p. F273.
Houghton, H. F., 1980, Refined techniques for staining
plagioclase and alkali feldspar in thin section: Journal
of Sedimentary Petrology, v. 50, p. 629–631.
Hutchison, C. S., 1989, Geological evolution of Southeast
Asia: Oxford, UK, Oxford Science Publications, 368 p.
Ingersoll, R. V., Bullard, T. F., Ford, R. L., Grimm, J. P.,
Pickle, J. D., and Sares, S. W., 1984, The effect of
grain size on detrital modes: a test of the Gazzi-Dickinson point-counting method: Journal of Sedimentary
Petrology, v. 54, p. 103–116.
Johnson, S. Y., and Nur Alam, A. M., 1991, Sedimentation
and tectonics of the Sylhet trough, Bangladesh: Geological Society of America Bulletin, v. 103, p. 1513–1527.
Kumar, P., and Uddin, A., 2004, Chrome-spinel constraints on provenance history of Cenozoic sediments
from Assam, northeast India [abs]: Geological Society
of America Annual Proceedings, Abstracts With
Programs, v. 36, p. 505.
Le Fort, P., 1996, Evolution of the Himalaya, in Yin, A.,
and Harrison, M., eds., The tectonic evolution of Asia:
New York, NY, Cambridge University Press, World and
Regional Geology Series, p. 95–109.
Mitchell, A. H. G., 1993, Cretaceous–Cenozoic tectonic
events in the western Myanmar (Burma)–Assam
region: Journal of the Geological Society, London, v.
150, p. 1089–1102.
Murphy, R. W., 1988, Bangladesh enters the oil era: Oil
and Gas Journal, February, p. 76–82.
Nagappa, Y., 1959, Foraminiferal biostratigraphy of the
Creataceous: Eocene succession in the India-PakistanBurma region: Micropalaeontology, v. 5, p. 145–192.
Najman, Y., and Garzanti, E., 2000, Reconstructing early
Himalayan tectonic evolution and paleogeography
from Tertiary foreland basin sedimentary rocks, northern India: Geological Society of America Bulletin, v.
112, p. 435–449.
Packham G., l996, Cenozoic SE Asia: Reconstructing its
aggregation and reorganization, in Hall, R., and
Blundell, D., eds., The evolution of Southeast Asia:
Geological Society (London) Special Publication No.
106, p. 123–153.
Patzelt, A., Li, H., Wang, J., and Appel, E., 1996, Palaeomagnetism of Cretaceous to Tertiary sediments from
southern Tibet: Evidence for the extent of the northern
margin of India prior to the collision with Eurasia:
Tectonophysics, v. 259, p. 259–284.
Pivnik, D. A., Nahm, J., Tucker, R. S., Smith, G. O., Nyein,
K., Nyunt, M., and Maung, P. H., 1998, Polyphase
deformation in a fore-arc/back-arc basin, Salin Subbasin, Myanmar (Burma): American Association of
Petroleum Geologists Bulletin, v. 82, p. 1837–1856.
Rangarao, A., 1983, Geology and hydrocarbon potential of
a part of Assam-Arakan basin and its adjacent area:
Petroleum Asia Journal, v. 6, p. 127–158.
Rowley, D. B., l996, The age of initiation of collision
between India and Asia: A review of stratigraphic data:
Earth and Planetary Science Letters, v. 145, p. 1–13.
Saikia, M. M., 1999, Indo-Burman orogenic belt: its plate
tectonic evolution, in Verma, P. K., ed., Geological
studies in the eastern Himalayas: Delhi, India,
Pilgrims Book (Pvt) Ltd., p. 19–39.
Sengupta, S., Ray, K. K., Acharyya, S. K., and De Smith,
J. B., 1990, Nature of ophiolite occurrences along the
eastern margin of the Indian plate and their tectonic
significance: Geology, v. 18, p. 439–442.
Sikdar, A.M., 1998, Tectonic evolution of eastern folded
belt of Bengal Basin: Unpubl. Ph.D. thesis, Dhaka
University, Dhaka, Bangladesh, 175 p.
Sinha, R. N., and Sastri, V. V., 1973, Correlation of the
Tertiary geosynclinal sediments of the Surma valley,
Assam and Tripura State (India): Sedimentary Geology,
v. 10, p. 107–134.
Uddin, A., Kumar, P., Sarma, J. N., and Akhter, S. H.,
2007, Heavy-mineral constraints on provenance of
Cenozoic sediments from the foreland basins of
Assam, India and Bangladesh: Erosional history of the
eastern Himalayas and the Indo-Burman ranges, in
Mange, M. A., and Wright, D. T., eds., Heavy minerals
in use: Amsterdam, The Netherlands, Elsevier, Developments in Sedimentology, v. 58, in press.
Uddin, A., and Lundberg, N., 1998a, Cenozoic history of
the Himalayan-Bengal system: Sand composition in
the Bengal basin, Bangladesh: Geological Society of
America Bulletin, v. 110, p. 497–511.
Uddin, A., and Lundberg, N., 1998b, Unroofing history of
the Eastern Himalaya and the Indo-Burman ranges:
Heavy-mineral study of Cenozoic sediments from the
Bengal basin, Bangladesh: Journal of Sedimentary
Research, v. 68, p. 465–472.
Uddin, A., and Lundberg, N., 1999, A paleo-Brahmaputra? Subsurface lithofacies analysis of Miocene deltaic
sediments in the Himalayan-Bengal system, Bangladesh: Sedimentary Geology, v. 123, p. 227–242.
Uddin, A. and Lundberg, N., 2004, Miocene sedimentation and subsidence during continent-continent collision, Bengal Basin, Bangladesh: Sedimentary Geology,
v. 164, p. 131–146.
Yin, A., and Harrison, M. T., 2000, Geologic evolution of
the Himalayan-Tibetan orogen: Annual Review of
Earth and Planetary Science, v. 28, p. 211–280.
Zutshi, P. L., 1993, Tectonics and hydrocarbon prospects of
Cachar-Tripura, eastern region, India: Bulletin of Oil
and Natural Gas Corporation Ltd., v. 30, p. 97–123.