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, northeastAssam 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 theEocene Disang Group (Qt68F3L29; total quartz–feldspar–lithic fragments) and the lower OligoceneNaogaon Formation (Qt69F6L25) are compositionally and texturally immature, composed mainly ofquartz, sedimentary and low-grade- metamorphic lithic fragments (including abundant chert), andplagioclase. Sandstones of the overlying middle and upper Oligocene Baragolai (Qt66F12L22) andTikak Parbat (Qt82F4L14) formations are similar but also contain significant amounts of volcanic andhigher 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 dep-osition of the middle and upper Oligocene sandstones. In contrast, Eo-Oligocene strata (Eocene:Qt99F1L0; Oligocene: Qt90F3L7) from the neighboring Bengal Basin contain angular quartzose sandsthat represent first-cycle detritus, most likely from the Indian craton. The Bengal Basin was pro-tected from orogenic sedimentation during the Eocene–Oligocene, either by a barrier to sedimenttransport (a peripheral forebulge or a marine basin) or by distance, prior to the approach of the basintoward Asia. Motion of this part of the Indian plate relative to now-adjacent Southeast Asia was mostlikely accomplished along strike-slip faults, like the N-S–trending Kaladan fault, located just eastof the Bengal Basin. Similarity in modal composition (quartzolithic to phyllarenitic) of Paleogenesequences of Assam and basins south of the Himalayan western syntaxis suggests that the Hima-layan emergence was not strongly diachronous, with initial collision and uplift at both syntaxialareas occurred in the Eocene.

Introduction

THE COLLISION OF India with Eurasia provides aspectacular lesson in plate tectonics. Timing of thecollision near the eastern syntaxis is very poorlyknown (Packham, l996; Rowley, l996), however, andimproved resolution on the timing would aid indeveloping more accurate models for deformation inthe eastern Himalayas. Data bearing on the timingof collision come mainly from areas west of the cen-tral Himalayas. Although most workers suggest thatIndia began to collide with Eurasia at around 50 Ma,others propose an earlier collision at about 70 Ma

(Yin and Harrison, 2000). Even less well understoodis the location of the boundary between India andIndochina through time. Most workers place themain boundary between India and Indochina for thepast 13 million years along the Sagaing fault inMyanmar (formerly Burma; Mitchell, 1993; Fig. 1).Total displacement on the Sagaing fault is not wellknown, but evidence on offset of ophiolitic rocks,and on opening of the Andaman Sea suggest about400 to 500 km of right slip (Curray, 1989). NUVEL-1A plate reconstructions place Assam, the northeastcorner of India, some 3000 ± 250 km south ofEurasia at about 50 Ma, and more recent recon-structions decrease this by only a few hundred kilo-meters (Gordon et al., 1999).1Corresponding author; email: uddinas@auburn.edu

0020-6814/07/950/798-13 $25.00 798

PALEOGENE SANDSTONES FROM ASSAM, NORTHEAST INDIA 799

Evidence of the early collision in the eastern Hima-layas should be recorded in the stratigraphic recordof basins south of the mountain belt. Paleogenestrata of the deep-sea Bengal fan have not yet beenrecovered (only back to about 18 or 17 Ma; Cochran,1990). Paleogene sandstones of the onshore delta ofthe Bengal Basin are quartzose, suggesting deriva-tion most possibly from non-orogenic sources(Uddin and Lundberg, 1998a). More proximal to theeastern Himalayas is the Assam Basin of India, aforeland basin with over 6 km of Eocene to Pleis-tocene marine to terrestrial strata deposited oncontinental crust. Thus it is anticipated that theinitiation of collision may be recorded by thesepredominantly non-marine or deltaic strata, in thatcollision likely began in the submarine realm. How-ever, considering the modern Taiwan collision, itappears 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 non-marine sediments were deposited on continentalcrust of the downgoing plate within 1 m.y. or so fromthe inception of collision (Covey, 1986). Further-more, it is important to note that the Assamsequence records the very early collision, becausethe initial detritus is rich in sedimentary lithic frag-ments and it subsequently shifted to dominance bymeta-sedimentary lithic fragments.

This study reports modal analyses of Eocene andOligocene sandstones exposed near the Margherita-Changlang area of northeast Assam, India. Compo-sitional data were collected to constrain the prove-nance of these deposits, and to compare them withcoeval sequences elsewhere in the foreland; thisshould help decipher the early erosional record ofthe eastern Himalayas in order to further elucidate

FIG. 1. Map of South Asia showing lithotectonic belts of Himalayan and Indo-Burman orogens and locations ofAssam and the Bengal Basin, along with other reference locations mentioned in the text. The Indian shield and ShillongPlateau expose Precambrian crystalline rocks. Approximate limits of the Indus and Bengal fan are shown. Deep SeaDrilling Project sites 217, 218, 222, 223, and 224 and area drilled by Ocean Drilling Program Leg 116 are shown in theBengal and Indus fans. Framed area is shown in detail in Figure 2 (after Uddin and Lundberg, 1998a).

800 UDDIN ET AL.

the history of collision between northeast India andAsia.

Regional Geologic Setting

The Assam Basin is bounded by the Indian cra-ton and the Shillong Plateau, a Precambrian massif,to the west; by the eastern segment of the Himalayasto the north; the Mishmi Hills in the northeast; theIndo-Burman Ranges to the east and immediatesouth; and the Bengal Basin of Bangladesh and theBengal deep-sea fan to the southwest (Fig. 1). Theeastern Himalayan syntaxis is located only about150 km NNE of Assam; parts of the Himalayas andthe Indo-Burman Ranges are located even closer(Fig. 2). The northernmost extension of the Indo-Burman Ranges merges with the E-W–trendingHimalayas at the Eastern Himalayan syntaxis. The

Himalayas consist of six longitudinal lithotectonicunits juxtaposed along generally N-dipping thrustfaults (Le Fort, 1996). From north to south (Fig. 1),these are the: (1) Trans-Himalayas, consisting ofcalc-alkaline plutons; (2) Indus suture zone, expos-ing ophiolitic bands representing the zone of colli-sion between India and Eurasia; (3) TibetanHimalayas, represented by fossiliferous Cambrianto Eocene sediments; (4) Higher Himalayas, locatednorth of the Main Central Thrust, composed ofschists, gneisses, and leucogranites; (5) Lower orLesser Himalayas, composed of unfossiliferous Pre-cambrian and Palaeozoic sedimentary rocks, andcrystalline rocks; and (6) Sub-Himalayas, represent-ing Miocene to Pleistocene molasse-type deposits ofthe Siwaliks. The N-S–trending Indo-BurmanRanges east and south of the Assam-Bengal systemconsist of early Tertiary synorogenic sediments,

FIG. 2. Map showing the Assam and Bengal basins and their tectonic elements such as the eastern Himalayas andIndo-Burman Ranges. Areas enclosed by the Naga and Disang thrusts form the Schuppen belt. Samples for this studywere 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 ShillongPlateau from the Sylhet trough of the Bengal Basin. The Kaladan fault, located east of the Chittagong Hills of the BengalBasin, separates the Assam sequences from the Bengal Basin (after Hutchison, 1989).

PALEOGENE SANDSTONES FROM ASSAM, NORTHEAST INDIA 801

schists, and ophiolitic belts (Fig. 1; Brunnschweiler,1966; Sengupta et al., 1990). Crystalline rocks,predominantly gneisses of Precambrian age, makeup the bulk of the Indian craton that is sporadicallyoverlain by Permian Gondwana deposits and Creta-ceous flood basalts of the Rajmahal Traps (Hutchi-son, 1989). Crustal material of a pre-Gondwanalandmass crops out in the Mikir Hills, the ShillongPlateau, and the Mishmi Hills, most of which lieoutside Assam. The Shillong Plateau, which is amajor geomorphic feature in the region, was upliftedto its present height in the Pliocene (Johnson andNur Alam, 1991). The southern edge of the plateauis bounded by the Dauki fault (Fig. 2; Uddin andLundberg, 2004).

Several thrust faults bound the Margherita-Changlang area of northeast Assam, including theNaga thrust to the northwest and Disang thrust to thesoutheast (Fig. 2). This thrust-bounded area is alsocalled the “Schuppen belt” (Rangarao, 1983). TheNaga thrust is a major décollement in the study area.Thrusting began in the late Eocene or earlyOligocene and continued into the late Pliocene; totalshortening is estimated to be about 300 km (Evans,1964; Saikia, 1999). The imbricate belt of the Nagathrust developed through compression during sub-duction (Fig. 2; Saikia, 1999). Geomorphically, theAssam and Bengal basins are separated by the MikirHills, the Shillong Plateau, and the Schuppen belt.

Thick successions of Cenozoic basin fill havebeen drilled and exposed in the Sylhet trough of theBengal Basin and uplifted along the Chittagong foldbelts of the eastern Bengal Basin (Fig. 2). The Chit-tagong fold belts comprise tight NNW-trending foldsalong the eastern edge of the foredeep. The Kohima-Patkai synclinorium is developed in the southernand southeastern parts of the Schuppen belt, andextends to the folded belt of the Sylhet trough andChittagong Hills (Fig. 2; Dasgupta, 1984). Thesefold belts represent a series of N-S–trending anti-clinal ridges and synclinal valleys, an arcuate beltthat is convex toward the west. The fold belt showsan increase in structural complexity toward the east,into the Arakan Yoma–Chin Hills and the Indo-Burman Ranges (Fig. 2). The latter are bounded bytwo N-S–trending right lateral faults, Sagaing to theeast and Kaladan to the west, adjacent to the BengalBasin (Uddin and Lundberg, 2004). Although theSagaing fault is commonly recognized as a right-lateral fault in Southeast Asia (e.g., Curray, 1989;Mitchell, 1993; Uddin and Lundberg, 1998a), theKaladan fault is not that popularly known. Although

this has a thrust component (Sikder, 1998), designa-tion of the Kaladan fault as a right-lateral one hasbeen promoted by Murphy (1988) and Zutshi(1993). The Kaladan fault trends NE-SW along theKaladan River, between the eastern boundary ofBangladesh and western Myanmar (Fig. 2; Murphy,1988; Zutshi, 1993; Sikdar, 1998). This fault istraceable on satellite images from the Disang thruston the north to offshore Myanmar on the south, adistance of few hundred kilometers.

Assam Paleogene Sequences

The stratigraphic framework of Assam is basedmainly on biostratigraphy, predominantly usingpalynology, with correlations depending on litho-stratigraphy (Evans, 1964; Sinha and Sastri, 1973;Rangarao, 1983). The basin sequences have alsobeen correlated by seismic stratigraphy by variousindustry groups, including the Oil and Natural GasCommission of India (Saikia, 1999).

The Paleogene section of the Margherita-Chang-lang area used in this study (Table 1) comprises theupper Eocene Disang Group (up to 3 km thick), thelower Oligocene Naogaon Formation (up to 2.2 km),the middle Oligocene Baragolai Formation (up to3.3 km), and the upper Oligocene Tikak ParbatFormation (~0.7 km; Table 1). The Oligocene forma-tions make up the Barail Group. The thickness ofeach unit decreases generally to the west (Rangarao,1983).

The Disang Group is marine, based on marinefossils, radiolarian cherts, and other typical deep-marine deposits. The Disang sequence consists offissile, carbonaceous mudrocks with fine-grainedsandstone. Nagappa (1959) reported arenaceous for-aminifera from the top part of Disang and suggesteda late Eocene age. Evans (1964) found Nummulitesfrom sandy shale of Disang and suggested a lateEocene age. The upper part of the Disang representsan argillaceous facies analogous to the EoceneSylhet and Kopili formations (shelf equivalents inthe Upper Assam Plains and Mikir hills; Rangarao,1983).

The Naogaon Formation consists mostly of fine-grained sandstones with subordinate siltstones,claystones, and shales, showing flaser and lenticularbedding. The middle unit of the Baragolai Forma-tion is dominantly argillaceous with thin siltstonesand sandstones. Shales in this unit are dark grey andcommonly show concretions. The Tikak Parbat For-mation is composed dominantly of grey, moderately

802 UDDIN ET AL.

sorted sandstones; minor siltstones and thick coalbeds are also present in this unit. These Oligoceneunits have been interpreted as brackish-water anddeltaic deposits (Rangarao, 1983).

Methods

Twenty-three representative Eocene–Oligocenesandstone samples from Assam were selected formodal analysis on the basis of appropriate grain sizeand low alteration. Most of the samples are highlyindurated. A few unconsolidated sand sampleschosen were sieved, and the fractions coarser than0.063 mm were epoxied into plugs for thin-sectionpreparation. Petrographic analyses were conductedfollowing the Gazzi-Dickinson method, countingsand-sized minerals included in lithic fragments asthe mineral phases rather than the host lithic frag-ment (i.e., Ingersoll et al., 1984). All thin sectionswere stained for plagioclase and potassium feldspar,following techniques modified from Houghton(1980). At least 300 framework points were countedper sample, with 400 framework points counted forsamples with greater compositional diversity.Selected thin sections were also counted a secondtime by a different person in order to evaluate oper-ator error.

Point-counting parameters and recalculatedparameters are defined in Table 2. Normalizedmodal data are given in Table 3 and representativephotomicrographs are shown in Figure 3. Polygonssurrounding mean values are calculated as samplestandard deviations, although these do not representtrue standard deviations for constrained-sum data(see Ingersoll et al., 1984); they are shown to indi-

cate the variability of values for each group. Ternarydiagrams using major detrital components, mono-crystalline grains, and the phaneritic lithic frag-ments were constructed in order to visualizevariations in sand composition and to help interpretthe tectonic provenance (i.e., Dickinson, 1985).Normalized modal data are depicted graphically inFigure 4A and 4B.

Assam Paleogene Sandstone Compositions

Modal analytical data from Eocene–Oligocenesequences in Assam are summarized below for thevarious stratigraphic units, from oldest to youngest.

Disang Group

Sandstones from the Eocene Disang Group(Qt68F3L29; Figs. 3A, 4A, and 4B) are composed offine- to medium-grained, subangular to angulargrains, containing mostly monocrystalline quartz,and also foliated and equant polycrystalline quartz,plagioclase, sedimentary and metamorphic lithicfragments of phyllite grade and fine-grained quartz-mica-chlorite schist. Sedimentary and low-grademetasedimentary lithic fragments suggest derivationof sediments from proximal orogenic sources. Likethe plagioclase, the large angular monocrystallinequartz could have been derived from a volcanicsource, or possibly from a granitic source, althoughthe almost complete lack of alkali feldspar suggestsotherwise.

Naogaon Formation

Sandstones from the lower Oligocene NaogaonFormation (Qt69F6L25; Figs. 3B, 4A, and 4B) are

TABLE 1. Paleogene Stratigraphy of the Margherita-Changlang Area in Upper Assam

Chronostratigraphy Group Formation Thickness (m) Brief lithology

Oligocene Barail Tikak Parbat 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

Late Eocene Disang Disang 2000 to 3300 Fine-grained sandstone with subordinate dark-gray shale rich in carbonaceous mat-ter and massive siltstone with concretions

Source: After Sinha and Sastri, 1973 and Rangarao, 1983.

PALEOGENE SANDSTONES FROM ASSAM, NORTHEAST INDIA 803

quartzolithic and contain subangular to angulargrains of monocrystalline and polycrystalline quartz,mostly plagioclase feldspar, and sedimentary andmetamorphic lithic fragments. Lithic fragments inthis unit are more diverse compared to the EoceneDisang Group sandstones.

Baragolai Formation

Sandstones from the middle Oligocene BaragolaiFormation (Qt66F12L22; Figs. 3C, 4A, and 4B) com-prise mono-and polycrystalline (also sheared) quartz,feldspar (mostly plagioclase, with chlorite and epidoteinclusions), sedimentary lithic fragments of shale,argillite and siltstone, and metamorphic lithic frag-ments of phyllite grade, fine- to medium-grainedquartzose-mica schists, and chlorite-quartz-epidote-zoisite schists. Chert grains are abundant (Fig. 3C).Volcanic lithic fragments are also present, mostly ofmafic lithologies with lathwork and local microlitictextures. Some of these volcanic lithic fragments ofBaragolai sandstones show massive alteration to chlo-rite and possible epidote, probably representing amild metamorphic overprint, although some alterationduring burial diagenesis may also have occurred.

Lower and middle Oligocene sandstones also suggesta proximal orogenic source because the detritus iscomposed of sedimentary, metasedimentary, volcanic,and metavolcanic lithologies. Sheared quartz grainsapparently were derived from zones of deformation. Aswith the older Disang unit, the lack or near absence ofalkali feldspar suggests no significant granitic sourcerocks for the lower Oligocene Naogaon Formation.Lath-shaped plagioclase grains probably representvolcanic phenocrysts. Chlorite-quartz-epidote (zoisite)schists may have been derived from low-grade meta-morphism of calcareous shales or mafic volcanicrocks. Rare grains of amphibole also suggest amedium-grade metamorphic source.

Tikak Parbat Formation

Sandstones from the upper Oligocene TikakParbat Formation (Qt82F4L14; Figs. 3D, 4A, and 4B)are texturally immature, with angular to subangularfragments, and are coarser than the older units.These upper Paleogene sandstones are also compo-sitionally immature, consisting primarily of grainscontaining monocrystalline quartz showing unduloseextinction. Sheared quartz, quartz-mica schist,

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.

804 UDDIN ET AL.

TABLE 3. Normalized Modal Analyses of Paleogene Sandstones from Assam, India

QtFL (%) QmFLt (%) QmPK (%) QpLvmLsm (%) LsLvLm (%)

Sample number Qt F L Qm F Lt Qm P K Qp Lsm Lvm Ls Lv Lm

Tikak Parbat Fm. (Upper Oligocene)

T-17 95 1 4 59 1 40 99 1 0 90 8.9 0.6849 43 7 50

T-12 78 8 13 42 8 49 83 11 6 73 27 0 62 0 38

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

Naogaon Fm. (Lower Oligocene)

NB-2 73 3 24 66 3 31 96 4 0 22 77 1 15 1 83

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

Disang Group (Eocene)

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

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

chlorite-mica schist, black shale, and polycrystal-line quartz (equant and foliated) are also present.These sandstones also contain abundant stretchedquartz grains, chert, epidote, muscovite, and biotite.Feldspars are sparse, and actinolite and epidoteschists are also rare. The presence of quartz-micaschists, chlorite-mica schist, and the abundance ofdetrital mica in the upper Oligocene to Neogenesandstones suggest a low- to intermediate-grademetamorphic source for the sandstones.

Interpretation of Assam PaleogeneSandstone Modes

All Paleogene (Eocene and Oligocene) unitsanalyzed plot in the “recycled orogenic” provenancefields of QtFL and QmFLt diagrams (Fig. 4A; Dick-

inson, 1985). These sandstones are quartzolithic(Table 3; Q71F6L23) and phyllarenitic, and containmore sedimentary and metasedimentary lithic frag-ments (Ls51Lv4Lm45). In the monocrystalline QmPKdiagram, most of the samples plot near the Qm pole(Fig. 4A). Volcanic components are generally scarcein Assam sandstones, with a peak in abundance inthe middle Oligocene Baragolai Formation that hashigher feldspar contents (Figs. 4A and 4B). SampleT-17, which has a very quartzose composition, wascollected from strata that are probably transitionalbetween the Oligocene Tikak Parbat Formation andthe Neogene Surma Group (Table 3). For reference,the Paleogene sandstones from the Bengal Basin arealso plotted in Figure 4 (Eocene—Be; Oligocene—Bo). The sandstones from the Bengal Basin show

FIG. 3. Representative photomicrographs of sandstones from Assam, India. A. Eocene Disang Group: frameworkgrains 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), plagio-clase feldspar (plag), and chert grains. C. Middle Oligocene Baragolai Formation: monocrystalline (Qm) and polycrys-talline 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 theseframework grains suggest orogenic derivation. In contrast, the Oligocene Barail Formation from the Bengal Basin showssubangular quartz grains with rare or no feldspar and lithic fragments (Fig. 6A; Uddin and Lundberg, 1998a).

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 eachstratigraphic unit. Provenance fields are from Dickinson (1985). For comparison, distribution of the Paleogene BengalBasin samples (Be = Bengal Basin Eocene; Bo = Bengal Basin Oligocene) is also shown in all the diagrams (from Uddinand 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 notstrictly valid statistically for constant-sum, constrained compositional data, polygons are shown to indicate ranges ofvalues. 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 deviationpolygons for each stratigraphic unit. These plots do not show presence of much volcanic lithic fragments except the mid-Oligocene Baragolai unit. For comparison, distribution of the Paleogene Bengal Basin samples (Be = Bengal BasinEocene; Bo = Bengal Basin Oligocene) is also shown in all the diagrams (from Uddin and Lundberg, 1998a). All theseplots show dominance of sedimentary and metamorphic lithic fragments in the Paleogene sequences of Assam. Volcaniclithic fragments are not that significant, except in the mid-Oligocene Baragolai unit.

PALEOGENE SANDSTONES FROM ASSAM, NORTHEAST INDIA 807

more maturity (placed close to the quartz poles thanthe Paleogene sandstones of Assam.

Paleogene Sandstones across the Himalayan Foreland

Paleogene sandstones from Assam are composi-tionally quite different from coeval sandstones of theadjacent deltaic Bengal Basin, but similar to coevalsandstones of the foreland basins south of the west-ern Himalayas. Eocene–Oligocene sandstone(s)from the Bengal Basin are less indurated and aredominantly quartzose (Qt90F3L7 to Qt99F1L0). Manyof the quartz grains are coarse and most are suban-gular to angular (Fig. 6A of Uddin and Lundberg,1998a). Almost all are monocrystalline grains, withvery minor polycrystalline grains, and sedimentarylithic fragments, scarce metamorphic lithic frag-ments, and no identifiable volcanic detritus. All ofthe rare feldspar grains are potassium feldspars.These quartz arenites are interpreted to have beenderived from the adjacent Indian craton (Uddin andLundberg, 1998a). The abundance of quartz andscarcity of both feldspar grains and lithic fragmentsin Bengal Basin sandstones also suggest a possiblesource terrane with low relief, with intense chemicalweathering due to the position of the basin close tothe equator during the Paleogene. Given sufficientlyintense chemical weathering, the possibility alsoexists that these quartzose sandstones were derivedfrom an orogenic source (Uddin and Lundberg,1998a). In more close proximity toward the north-west of the Bengal Basin and west of Assam, but stillin the eastern half of the Himalayas, in the westernand central Nepal, the Paleocene fluvial to shallow-marine Amile Formation and Eocene marine to shal-low-marine Bhainskati Formation are pure quartz-arenites (Fig. 1; DeCelles et al., 1988). The lowerMiocene nonmarine Dumri Formation in westernNepal is quartzolithic (Qt72F4L24; DeCelles et al.,1998) with very little feldspar, most of which isplagioclase. Zircon dates from these units suggest apossible Himalayan source (DeCelles et al., 1998).

In the western Himalayan basins, the upperPaleocene to lower Miocene synorogenic sedimentsthat began to fill the evolving foreland basins thatdeveloped ahead of the southward-advancing Hima-layas comprise terrestrial sediments of the lithofelds-pathic Chulung La Formation (Fig. 1; Paleocene toOligocene; Qt24F26L50), quartzolithic tidal-flat tofluviatile deposits of the Murree Supergroup (Paleo-cene to Oligocene; Qt68F5L27; Garzanti et al., 1987;

Critelli and Garzanti, 1994), the quartzolithic shal-low-marine Subathu Formation (upper Paleocene tomiddle Eocene; Qt63F7L30; Najman and Garzanti,2000), and tidal flat to alluvial quartzolithic DagshaiFormation (upper Oligocene; Qt58F1L31; Najmanand Garzanti, 2000; Fig. 1). Like the sandstonesfrom Assam and unlike the sandstones from theadjacent Bengal Basin, these units contain abun-dant metasedimentary, volcanic, and sedimentarylithic fragments and ophiolitic detritus, beginningearly in the Paleogene (Garzanti et al., 1987; Critelliand Garzanti, 1994).

Early Orogenic Historyof the Eastern Himalayas

The Eocene–Oligocene sandstones from Assamwere clearly derived from an orogenic source,exposing and eroding sedimentary and low-grademetamorphic units to form the older sandstones,followed by increasing contributions from volcanicand higher grade metamorphic rocks during deposi-tion of the middle and upper Oligocene sandstones(Fig. 5A). The Assam sandstones provide clearevidence that orogeny had begun in the easternHimalayas by the Eocene, in contrast to the earlyMiocene initiation suggested by the apparently first-cycle Paleogene quartz arenites (Uddin and Lund-berg, 1998a, 1998b) and subsurface lithofaciespatterns of Miocene (Uddin and Lundberg, 1999) ofthe Bengal Basin. The more proximal Assamsequence apparently records the early stages oforogenic activity; whereas the initial detritus is richin sedimentary lithic fragments, later sandstonesshow a subsequent shift to dominance by metasedi-mentary lithic fragments.

Heavy-mineral contents in Oligocene sequencesfrom Assam are composed mostly of zircon, tourma-line, and rutile (ZTR) that are also associated amongothers with chloritoid, epidote, garnet, hornblende,kyanite, staurolite, and spinel, suggesting an oro-genic source (Uddin et al., 2007). Microprobe studyof garnets and chrome-spinel grains from Paleogenesequences of Assam also suggest a Himalayansource material (or ophiolites) and/or the Indo-Burmese ophiolitic belts (Kumar and Uddin, 2004).Presence of dominantly ZTR minerals among thenonopaque variety in the Eocene and Oligocenesequences of the Bengal Basin suggests intensepost-depositional weathering and does not obviouslysuggest an orogenic source (Uddin and Lundberg,1998b). Heavy-mineral assemblages in both the

808 UDDIN ET AL.

Assam and Bengal basins become more diverse inMiocene and younger formations, indicating deriva-tion from orogenic belts (Uddin et al., 2007).

The Bengal Basin may have been protected fromorogenic sedimentation during Eocene and Oligo-cene time, either by a barrier to sediment transport(a peripheral forebulge, or a marine basin, for exam-ple) or simply by distance (Fig. 5A). Early uplifts ofthe Indo-Burman Ranges could potentially have

acted as a barrier; however, that seems unlikelybecause the westward-encroaching ranges wereprobably located farther east relative to the BengalBasin during the Paleogene than in the Miocene(Mitchell, 1993; Uddin and Lundberg, 1999). Thesecompositional data also suggest that the Assam andBengal basins were latitudinally farther apart priorto early Miocene time, and as a consequence, werereceiving detritus from two distinct sources. The twosequences are presently exposed on either sideof the N-S–trending right-lateral Kaladan fault(Murphy, 1988; Zutshi, 1993), between the easternfold belts of the Bengal Basin and western folds inAssam, India (Figs. 1 and 2). This transpressionalfault seems to be resulting from oblique conver-gence of India with Indochina. India has beenmoving both north and eastward; the northerlymotion has been attributed to the Miocene openingof the Andaman Sea resulting in N-directed move-ment of India along right-lateral faults (Pivnik et al.,1998). A strong candidate for such a fault is theKaladan fault (Uddin et al., 2007). These two dis-tinct sequences were in close proximity by earlyMiocene time because both are covered by lowerMiocene strata (the Bhuban Formation of the lowerSurma Group; Johnson and Nur Alam, 1991; Uddinand Lundberg, 2004) that are similar in provenance(Fig. 5B; Godwin et al., 2001; Uddin et al., 2007).

More regionally, the thick Eocene–Oligocenesands from the Assam sequences are similar in com-position to those of the western Himalayan foreland,suggesting that the initial collision of Northeast andNorthwest India with Asia was not strongly diachro-nous. This non-diachronous convergence is alsosupported by isotopic and compositional studies(DeCelles et al., 1998) and paleomagnetic study(Patzelt et al., 1996) and by work on subsequentMiocene metamorphism and cooling history of thetwo syntaxial areas (Nanga Parbat in the west andNamche Barwa in the east; Ding et al., 2001). Thisassumes, however, that Assam was initially part ofIndochina.

The active Kaladan fault appears to (geographi-cally) separate the two Eocene–Oligocenesequences in Assam and the Bengal Basin. Detritusin the latter was apparently derived from the neigh-boring Indian craton, accumulating on crust of theIndian plate prior to arrival of the clastic wedgeshed from the approaching orogeny. If true, then theMiocene strata represent an overlap assemblage,signifying the “docking” of this part of the Indianplate with proximal terranes of Asia. One possible

FIG. 5. Schematic paleogeographic reconstruction of theHimalayan and surrounding areas during the Paleogene timeshowing tectonic elements of Assam, India, and Bengal Basinin (A) pre-Miocene and (B) Miocene time. The Bengal Basinmay have been transported close to Assam during theMiocene along right-lateral faults (i.e., the Kaladan fault)located east of the basin.

PALEOGENE SANDSTONES FROM ASSAM, NORTHEAST INDIA 809

explanation of the contrast in sediment source isthat the part of the Indian plate represented by theBengal Basin was still far to the south of Asia untilthe Miocene, when it arrived close enough to receivedetritus from the orogenic highlands fringing Asia’ssouthern boundary. It is possible that the sequencespreserved in the Bengal Basin and Assam were orig-inally deposited on two separate lithospheric plates,with Assam as part of Indochina (Fig. 5A).

Conclusions

Paleogene sandstone composition from the studyarea of northeastern Assam indicates recycledorogenic derivation. The Assam sandstones differfrom coeval sandstones in the adjacent BengalBasin, which are texturally immature first-cyclequartz arenites that were most likely derived fromthe neighboring Indian craton. The Bengal Basinwas probably protected from orogenic sedimentationduring the Paleogene, either by a barrier to sedi-ment transport or distance. If “distance” was thecause, then the part of the Indian continent repre-sented by the Bengal Basin was far to the south ofAsia until the early Miocene. Motion of this part ofthe Indian plate relative to Southeast Asia (Indo-china) was most likely accomplished along right-lateral faults, like the N-S–trending Kaladan fault,located east of Bangladesh.

If the analyzed Paleogene sequences of Assamwere deposited on Indian continental crust, then theHimalayan collision was not strongly diachronous,with initial collision of both Northeast and North-west India in the Eocene forming the two syntaxialbends of the Himalayas. This suggestion of non-diachroneity is also supported by similarity incomposition and thickness of Paleogene strata inbasins (Assam, India and Pakistan) near the twoHimalayan syntaxes.

Acknowledgments

Thanks to Suvrat Kher for confirmational pointcounts. Neil Lundberg, Clark Burchfiel, Roy Odom,and Suvrat Kher helped with various aspects of thework, including editing parts of the manuscript. Sev-eral students from Dibrugarh University (Assam)helped during field work and sample collection forthe project. Mr. T. Bordoloi helped with logistics inDigboi, Assam. Reviews by Peter DeCelles and RayIngersoll have significantly improved the manu-script. M. Shamsudduha and Khandaker Zahid

helped draft some figures. PK received a grant-in-aid support from Geological Society of America.This manuscript is supported by U.S. NationalScience Foundation grant EAR-0310306 awardedto AU.

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