The Dras arc Complex: lithofacies and reconstruction of a Late Cretaceous oceanic volcanic arc in...

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ELSEVIER Sedimentary Geology 92 (1994) 117-145

SEDIMENTARY GEOLOGY

The Dras arc Complex: lithofacies and reconstruction of a Late Cretaceous oceanic volcanic arc

in the Indus Suture Zone, Ladakh Himalaya

Alastair Robertson, Paul Degnan Department of Geology and Geophysics, Grant Institute, University of Edinburgh, West Mains Rd., Edinburgh EH9 3JW, UK

Received June 6, 1993; revised version accepted January 12, 1994

Abstract

The purpose of this paper is to give an integrated description and interpretation of mainly volcaniclastic sediments related to excellently exposed oceanic volcanic arc successions in the Ladakh Himalayas. The mainly Late Cretaceous (Aptian-Paleocene?) Dras arc Complex in the Indus Suture Zone (N. India) is reconstructed as an oceanic arc, passing southwards into a proximal to distal forearc apron. The arc complex comprises three structural units. From west to east these are the Suru unit, the Naktul unit and the Nindam Formation. The Suru unit and the Naktul unit are unconformably underlain by dissected Late Jurassic? oceanic crust and mantle. The Suru unit preserves the interior of the arc and is divided into Dras 1 and Dras 2 sub-units. The Dras 1 Sub-unit, of mid-Late Cretaceous age, was intruded by arc plutonics, deformed, then unconformably overlain by the poorly dated Dras 2 Sub-unit (Lower Tertiary). The Dras 1 Sub-unit comprises arc extrusives, volcaniclastic and tuffaceous sedimentary rocks, and mainly redeposited shallow-water limestones. The Dras 2 Sub-unit is dominated by coarse volcaniclastics and lava flows, passing up into rhythmically layered acidic extrusives, with interbedded turbiditic siltstones and siliceous pelagic limestones. Further east, the Naktul unit is mainly clastic, with large volumes of massive volcaniclastic talus, thick-bedded debris flows, volcaniclastic turbidites and reworked shallow-water carbonates. Pillowed extrusives and ribbon radiolarites are present, mainly low in the succession in some areas, while pelagic carbonates are abundant near the top. The Naktul unit is interpreted as a proximal forearc apron. The Nindam Formation in the east is dominated by deep-water volcaniclastic turbidites, tuffaceous sediments and pelagic carbonates, with subordinate debris flows and is interpreted as a distal deep-water forearc succession. Cyclical alternations of mainly volcaniclastics and pelagic carbonates in the Nindam Formation are thought to reflect relative sea-level changes, of tectono-magmatic a n d / o r global eustatic origin.

The oceanic Dras arc Complex formed within Neotethys in the mid-Late Cretaceous above an assumed northward-dipping subduction zone. Around 79 Ma (Campanian), the oceanic arc underwent collisional deformation, possibly related to accretion to the Trans-Himalaya (Ladakh Block) a n d / o r the southern margin of Eurasia, to form an Andean-type active continental margin. The Suru unit in the west (Dras 1 Sub-unit) was deformed, then overlain by the mainly acidic Dras 2 Sub-unit. However, there is no direct evidence that the Nindam Formation in the east was deformed until the Early Eocene when final collision of India and Eurasia took place, with southward thrusting of the Dras arc Complex. This was followed by partial erosion during deposition of the "Indus molasse", and then back-thrusting and folding related to uplift of the High Himalayas in the later Tertiary.

0037-0738/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0037-0738(94)00025-P

118 A. Robertson, P. Degnan / Sedimentary Geology 92 (1994) 117-145

1. Introduction

The object of this paper is to describe and interpret dominantly volcaniclastic sedimentation related to an oceanic arc exposed within the Indus Suture Zone of the Ladakh Himalaya, here termed the Dras arc Complex (Fig. 1). This oceanic arc was generated in the mid-Late Creta- ceous by inferred northwards subduction during the northward drift of India towards Asia. Based on facies and petrographic data, we will argue that intrusives, extrusives and volcaniclastic sediments, in several different tectonic units, can be reconstructed as a single oceanic arc and its prox-

imal to distal forearc apron. Indeed, outstanding exposure makes this an ideal area for the study of arc-related, volcaniclastic sedimentary processes. In the first part of the paper, the stratigraphy, regional tectonic setting and plate tectonic models are discussed, while in the second part (Sec- tions 4-7) the lithofacies, petrography and deposition are described and interpreted.

2. Regional setting, tectono-stratigraphy and age

The Dras arc Complex forms an up to 15 km-wide by 400 km-long outcrop in Western

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The Dras arc Complex is present as three separate structural units within the Indus Suture Zone, here termed the Suru unit (new name), the Naktul unit and the Nindarn Formation.

2.1. Suru unit

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Fig, 3. Representative cross-sections of the Indus Suture Zone in western Ladakh. (A) Suru and Naktul units of the Dras arc Complex in the west. (B) Central area, near Bodkhabu. (C) Nindam Formation of the Dras arc Complex in the east. (A, B) Modified after Sutre (1990). (C) This study, c = Central area. Locations of cross-sections are shown in Fig, 2.

Ladakh (Thakur, 1981, 1990; Fig. 2). The arc units are bounded to the north by the Trans- Himalayan unit (also known as the Ladakh Block and the Ladakh Batholith Terrane), and to the south by the Mesozoic Lamayuru Complex that comprises deep-water sedimentary and igneous rocks of the Indian passive margin (Bassoullet et al., 1978; Searle, 1983; Robertson and Degnan, 1993; Fig. 3). Thin melange units are present along both the southern and northern margins of the arc complex and locally within it (e.g. Thakur, 1981, 1990). The Dras arc Complex experienced southward thrusting in the Eocene related to final collision of India and Eurasia. However, the outcrop-scale structure is dominated by the effects of northwards backthrusting and folding related to the later Tertiary uplift of the High Himalayas to the south (e.g. Searle et al., 1988).

The Suru unit in the west (Figs. 2, 3A) preserves intrusives, extrusives and related sedimentary rocks of a typical island arc type, estimated as more than 5 km thick (Prasad et al., 1980; Thakur, 1981, 1990; Honegger, 1983; Sharma and Gupta, 1983; Riebel, 1984; Reuber, 1989). Map- ping by Reuber (1989) established that the Suru unit depositionally overlies older Neotethyan oceanic crust and mantle. In different areas, basal arc volcanics and sediments unconformably over- lie ultramafics, gabbros and/or pillow basalts, locally dated as Callovian-Tithonian, based on Radiolaria in cherts above pillow basalts southwest of Dras (Honegger, 1983). Reuber (1989) inferred that a phase of intra-oceanic compres- sion of Neotethyan crust and mantle had caused uplift and erosion: this possibly could relate to the onset of subduction. Alternatively, oceanic crust and mantle could have been exposed on the sea floor by tectonic extension and/or transform faulting, as, for example, observed at slow spreading ridges and oceanic fracture zones.

Reuber (1989) divided the Dras arc volcanics and sediments (our Suru unit) into a strongly deformed and metamorphosed lower unit ("Dras 1"), and a less deformed and unmetamorphosed upper unit ("Dras 2"). Here, these two units are termed, respectively, the Dras 1 and Dras 2 Sub- units of the Suru unit. Counterparts of the Dras 2 Sub-unit have not been identified in the Naktul unit, or the Nindam Formation. Volcanics and volcaniclastics of the Dras 1 Sub-unit are interbedded with limestones containing rudists and poorly preserved Orbitolina (Mangain and Jagan- natra Rao, 1965; Reuber, 1989). Also, a Middle Albian ammonite (Oxytropidoceras) was discov- ered south of Kargil (Thieuloy et al., 1990).

In Ladakh, the ages of intrusives within the Dras arc Complex and the adjacent Trans- Himalaya range from 110 to 10 Ma. The oldest, 110-90 Ma (Albian-Cenomanian) are in the

A. Robertson, P. Degnan /Sedimentary Geology 92 (1994) 117-145 121

south (Dras-Kargil area), followed by 80-60 Ma ages (Late Cretaceous) for syn-tectonic intrusions, thought to mark a collisional event, then 50-20 Ma, interpreted as being related to continental collision, and finally 20-10 Ma, viewed as resulting from melting and intrusion above the Main Central Thrust of the High Himalayas (Sharma et al., 1978; Honegger et al., 1982; Die- trich et al., 1983; Scharer et al., 1984; Reuber et al., 1989; Sharma et al., 1989). Ca. 100 and ca. 60 Ma U/Pb ages of Trans-Himalayan granodiorites (Scharer et al., 1984) are considered particularly reliable. The ca. 100 Ma U/Pb isotopic data also suggest the existence of a crustal component in granodiorites (near Kargil) cutting basaltic extrusives of the Dras arc (Scharer et al., 1984). This, however, is problematic in view of correlation with the inferred oceanic Kohistan arc to the west, as discussed below.

The metamorphism of the Dras 1 Sub-unit is most intensive in the north (near Kargil), marked by the development of amphibolites within shear zones, dated at 75 Ma by the K-Ar method (Reuber, 1989). This event could reflect collision of the oceanic Dras arc with the Trans-Himalaya and/or the Eurasian margin to the north. The age of the Dras 2 Sub-unit is constrained only as between a 60 Ma granite intrusion (Honegger et al., 1983) and the Miocene? age of the base of the unconformably overlying "Indus molasse" (Re- uber, 1989).

2.2. Problem of the "'Dras 1" and "Dras 2" Sub- units

In the west, the preserved interior of the arc (Suru unit) is made up of up to ca. 5 kin-thick successions of volcaniclastic sediments, shales, limestones, extrusives and intrusive rocks. These lithologies experienced greenschist facies metamorphism and higher-grade contact metamorphism in the vicinity of arc intrusives, mainly in the north (Fig. 4, 5e; Reuber, 1989).

Reuber (1989) mapped two sub-divisions of the Dras unit. On both sides of the Suru river valley, she recognised a deformed, steeply dipping "Dras 1" unit, overlain by a "Dras 2" unit (Fig. 4). For example, along the eastern side of

the Suru River Valley, south of Kunoore, numer- ous inclined, to recumbent folds in the "Dras 1" unit (mainly N-verging) contrast strongly with a well stratified, effectively undeformed, gently dipping "Dras 2" succession above. The contact is definitely an unconformity.

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Reuber (1989) also mapped the east-west- trending, elongate Stang and Dandaldung mountains on either side of the Suru Valley (e.g. south of Kunoore River) as part of her "Dras 2" unit, on the basis of their relatively undeformed and unmetamorphosed state, in contrast with "Dras 1" units to the north. These "Dras 2" units she described as mainly coarse, unstratified subaerial vent agglomerates ("chimney breccias"). She also stated that in the "Dras 2", "well layered facies grade into the massive breccias of Danduldung and Stang Mountains, in which not only volcanic and volcaniclastic material is reworked, but also some ultramafics and diorites". The ultramafics are assumed to have been derived from ophiolitic basement and the diorites from Dras 1 intrusives. Inaccessibility of these outcrops makes these key observations difficult to verify.

Correlation of the kilometre-thick, mainly breccia, units of the Stang and Dandaldung mountains with the contrasting well-layered "Dras 2" is, however, questionable for several reasons: (1) Reuber (1989) mapped subvertical fault con- tacts separating the "Dras 2" breccias (i.e. Stang and Dandaldung mountains) from the well- layered definite Dras 2 succession further north (e.g. south of Kunoore River; her fig. 13); (2) Reuber's (1989) excellent photograph (her fig. l la) of well-stratified Dras 2 lithologies unconformably overlying granodiorite (that cuts Dras 1 and ophiolitic basement lithologies) is of an isolated outcrop near Thasgam (Fig. 4), which is structurally and lithologically similar to the definite Dras 2 outcrop east of the Suru River (south of Kunoore, Fig. 4), but is unlike the massive breccias of the main Dandaldung Mountain outcrop further south; (3) our observations show that the Stang and Danduldung outcrops in the Suru Valley originated as subaqueous (radiolarian- bearing), proximal volcaniclastic talus, rather than subaerial vent agglomerates ("chimney breccias"); (4) in some areas (e.g. northeast of Stang Mtn.) the well-layered Dras 2 succession is steeply dipping, thrust-imbricated and folded, thus blurring any simple structural distinction between Dras 1 and Dras 2 in terms of structural style.

Summarising, we confirm the existence of a separate Dras 2 arc unit, locally unconformably

overlying a Dras 1 arc unit, but we interpret the thick breccias of the Stang and Danduldung mountains as originally forming part of the proximal southern margin of the Dras 1 arc unit, not as Dras 2 (Figs. 5d, 5e). These breccias remained relatively undeformed and metamorphosed be- cause of their massive, competent nature and southerly position, removed from arc plutonism, or the site of inferred collision in the north. The breccias were finally thrust northwards over the more deformed Dras 1 arc core to the north during the Late Tertiary backthrust event.

2.3. Naktul unit

The Dras unit is structurally overlain by the Naktul unit (Figs. 2, 5c), which is largely composed of volcaniclastic sediments, with subordinate subaqueous basic extrusives, limestones and minor cherts (Reuber, 1989; Sutre, 1990). In places, the Naktul unit depositionally overlies peridotites, cut by swarms of basic dykes. Sutre (1990) reported breccias containing clasts with rudists, Orbitolina and Foraminifera of Aptian- Cenomanian age within the overlying volcaniclastic succession, while pink limestones from the upper part of the succession (e. g. Wakka River and Naktul Mountain; Fig. 4) are dated as Late Maastrichtian by planktonic Foraminifera. Dykes cutting the basement peridotite are assumed to be feeders of the arc extrusives and these gave a K /Ar age of 96.9 plus or minus 6.5 Ma (Albian- Turonian; Reuber et al., 1989). Sutre (1990) interpreted the Naktul unit as a preserved oceanic volcanic arc centre, separate from his Dras unit in the west (Suru unit of this study). However, in this study the Naktul unit was found to be dominated by proximal volcaniclastics, suggesting an arc-marginal setting.

Further east, the eastward extension of the Naktul unit is separated from the Nindam For- mation to the south by a thin strip of serpentinitic melange. Reconnaissance during this study in the vicinity of Bodhkhabu (Section C-C', Fig. 3B) revealed successions of inverted pillowed and massive extrusives of mainly intermediate composition (directly south of a melange zone) which can be correlated with the Naktul unit. Higher

124 A. Robertson, P. Degnan /Sedimentary Geology 92 (1994) 117-145

parts of the succession further north were not accessible. The melange zone includes sheared lavas, volcaniclastic sediments, black shales and sandstones containing terrigenous material (e.g. mica-schist). South of the melange there is an exceptionally well exposed, thick, intact, northwards-younging succession consisting of volcaniclastic and pelagic sediments, correlated with the Nindam Formation (Fig. 5a). Sandstones in the lower part of this succession contain rare Or- bitolina, while Globotruncana is present in pelagic carbonates near the top of the succession. The two units in this area are therefore interpreted as proximal (in the north) and more distal (in the south) parts of the oceanic Dras forearc that were tectonically juxtaposed during Early Eocene collision and/or later Tertiary backthrusting.

2.4. Nindam Formation

In its type area further east (Fig. 2), the Nin- dam Formation is exposed as a several km-thick, well stratified northwards-younging succession, dominated by epiclastic volcanogenic sediments, water-lain tufts and pelagic carbonates. Sutre (1990) established Cenomanian-late Maas- trichtian ages for the in-situ sediments of the Nindam Formation in the east, based on Globotruncana, an ammonite (displaced) and In- oceramus sp. Clasts of shallow-water-derived limestone, additionally, yielded Valanginian- Cenomanian ages, based mainly on Orbitolina, Foraminifera (Pseodotextularia) and rudists (Sutre, 1990).

The Nindam Formation has been correlated with the "Dras arc" to the west (Bassoullet et al., 1978; Frank et al., 1979; Fuchs, 1979; Thakur and Misra, 1984; Colchen et al., 1985), or, alternatively, interpreted as part of the forearc of an Andean-type continental margin in the Trans- Himalaya to the north (Sutre, 1990). In this paper, we interpret the Nindam Formation as the distal forearc succession of a Late Cretaceous oceanic volcanic arc.

The Dras arc Complex originally was more extensive than the present outcrop would suggest. In the east, the outcrop of the Nindam Formation ends in the vicinity of the Zanskar River (Fig. 1).

However, further east in the Markha Valley the Eocene? base of the Indus molasse ("Dras Flysch" of Fuchs, 1986) contains abundant well- rounded clasts of mainly intermediate composition, extrusives and volcaniclastic sediments, that can only have been derived from a now unex- posed, eastward extension of the Dras arc Com- plex, including volcanics (not exposed in the Nin- dam Formation).

In the east, in the vicinity of Manlung (Fig. 10), the Upper Cretaceous Nindam Formation passes, apparently without a break, upwards into 100-200 m of thin-bedded, fine- to medium- grained volcaniclastic turbiditic silststones, interbedded with hemipelagic calcareous mudstones. Sutre (1990) states that these sediments lie between strata of latest Maastrichtian (based on Orbitoides) and Early Eocene age (based on Nummulites). He infers this succession to be of late Maastrichtian to basal Eocene age. The Nin- dam Formation thus seems not to have been deformed by collisional deformation during the Late Cretaceous, in contrast to the Suru unit in the west. The overlying Eocene Nummulites For- mation (Van Haver, 1934) is a generally thickening-, coarsening- and shallowing-upwards volcaniclastic and calcareous succession (of mainly similar provenance to the Nindam Formation) that includes shallow-water carbonates and conglomerates with granitic clasts, presumably derived from the Trans-Himalaya to the north.

The Nindam Formation is bounded to the north by a tectonic melange unit (Mongyu Melange), including serpentinite and Upper Cre- taceous terrigenous clastic sediments. This melange is interpreted as having formed by clo- sure of an oceanic area that separated the oceanic Dras arc Complex from an Andean-type margin in the Trans-Himalaya to the north (our unpublished data). To the south, the Nindam Forma- tion is locally in tectonic contact with another melange, which includes blocks of mid ocean ridge- and island arc-type basalts (Urtsi Melange; Fig. 10). This unit previously was interpreted as relatively in situ, Late Jurassic oceanic basement of the Nindam Formation (Sutre, 1990). Our mapping, however, shows that the oceanic crust is restricted to blocks in melange, which is in thrust


contact with the overlying Nindam Formation. This melange is instead interpreted as a subduction/accretion complex (Robertson and Degnan, in prep.).

3. Relation to other regional units

The Dras arc is widely correlated with the Cretaceous Kohistan arc of northern Pakistan (Fig. 1, inset), based on approximately similar ages (ca. 100-80 Ma) and lithologies, although there is no continuous outcrop. The deepest levels of the Kohistan arc are exposed in the south and include layered gabbros, with rare ultramafic cumulates (e.g. Jilal Complex), overlain by a thick metavolcanic sequence with basic intrusives and deformed quartz-diorites (Kamila Amphibolite Belt), followed, further north, by layered gabbro- norites and garnet granulites (Chilas Complex), together with very rare harzburgites and dunites (Coward et al., 1986; Petterson and Windley, 1991; Khan et al., 1993). Further north there is a thin strip of mainly paragneisses of meta-sedimentary origin (Gilgit Paragneisses; Khan et al., 1993), then calc-alkaline volcanics (Chalt Vol- canic Group) and sediments (Yasin Sedimentary Group); these possibly formed in an Albian- Cenomanian-aged back-arc basin (Pudsey et al., 1985b). In addition, Sullivan et al. (1993) report an 4°Ar/39mr age of 55 Ma from the Dir-Utror volcanics which were erupted after accretion of the oceanic arc to the Karakoram active margin.

Searle (1983) correlated orthopyroxene-bearing norites near Kargil with the Chilas Complex in Kohistan (see also Rai and Pande, 1978). How- ever, whereas the Chilas Complex is interpreted as part of the oceanic Kohistan arc, the isotopic evidence suggests a crustal component in the early, ca. 100 Ma intrusives in Ladakh (Scharer et al., 1984), possibly reflecting arc volcanism on, or near, a microcontinent. In Ladakh, the Dras 2 succession can be correlated with the proximal Utror Volcanic Formation (ca. 48 Ma) and the more distal Baraul Banda Slate Formation (ca. 60 Ma) (Sullivan et al., 1993), the Khardung Vol- canics (Srimal et al., 1978) and the Lingzizong unit of Tibet (Coulon et al., 1986). Each of these

units is dominated by siliceous extrusives, interpreted to have erupted along an Andean-type continental margin after accretion of the oceanic Dras arc.

In Ladakh, the Trans-Himalaya north of the Dras arc Complex is dominated by arc plutonics (Sharma and Choubet, 1983), ranging in age from Late Cretaceous to Late Tertiary. The country rock of the Ladakh Block (Trans-Himalaya) has recently been shown to include continental crust. Raz and Honegger (1989) inferred a succession of Upper Palaeozoic metaelastics overlain by Meso- zoic carbonates, based on: (1) a fragmentary Tri- assic-Liassic carbonate succession northeast of Khalsi (north of Likir Gompa; Raz and Honeg- ger, 1989); (2) Palaeozoic granites, 80 km east of Ley (ca. 235 Ma Gaik granite; Triverdi et al., 1981); (3) metamorphic rocks (including phyllites, schists and gneisses) and limestones, intruded by granites (Khalsar Formation), forming thrust sheets low in the structural succession in the Shyok suture north of the Trans-Himalayan batholiths in Ladakh (ca. 50 km north of Ley) (Rai, 1983; Thakur, 1990); (4) Upper Cretaceous (Albian-Aptian) shallow-water limestones (Khalsi Limestones) in tectonic contact with the southern margin of the Trans-Himalaya in central Ladakh (Khalsi area), interpreted as remnants of a forearc basin to a continental margin to the north (Van Haver, 1984; Sutre, 1990). The Ladakh Block can be interpreted as a continental fragment that possibly passed westwards into the Gilgit Parag- neisses in Kohistan. To the east the Ladakh Block wedges out along faults, but has a counterpart in the Lhasa Block further east. The Cretaceous- Early Tertiary Trans-Himalayan forearc succession can also be correlated with the Xigase Group in Tibet (Searle et al., 1987); this also contains abundant acidic terrigenous sediment (Einsele et al., 1993).

In the Late Cretaceous the Trans-Himalaya (Ladakh Block) was separated from the Karako- rum Batholith Terrane to the north by the North- ern (Shyok) Suture, interpreted as a Japan Sea- type marginal basin by Thakur (1981, 1990), but as a wider Neotethyan oceanic basin by others (e.g. Sutre, 1990). In Kohistan the Chalt volcanics and Yasin sediments were also interpreted as a


marginal basin: this closed by the end of the Cretaceous (Pudsey et al., 1985a; Pudsey, 1986), as indicated by a 75 Ma hornblende K - A r age on a post-tectonic diorite dyke (Petterson and Wind- ley, 1991).

One problematic aspect is that Upper Creta- ceous volcaniclastics of the Nindam Formation sampled on the road between Lamayuru and Khalsi (Fig. 10) possess a magnetisation component suggesting acquisition at a low southerly palaeolatitude, assuming a primary origin (Kloot- wijk et al., 1984). Secondary magnetisation components in intrusives from near Kargil yielded a similar result (Klootwijk et al., 1977). The Kohis- tan arc apparently also formed at equatorial lati- tudes based on palaeomagnetic data (Klootwijk et a l , 1985). Assuming that the Kohistan-Dras arc was amalgamated by the end of the Cretaceous with the Eurasian margin, either the palaeomag-

netic data are not primary, or a wide ocean must still have existed north of the Karakoram.

In summary, alternative plate tectonic models for the Kohistan-Dras arc Complex are shown in Fig. 6. The chief objection to Fig. 6a - - the Dras arc Complex as forearc to the Cretaceous Trans- Himalayan arc to the north - - is the correlation with the inferred oceanic Kohistan arc to the west and the strong contrast between the mainly intermediate to acidic composition of the Trans- Himalayan forearc succession, compared with the mainly basic-intermediate composition of the Dras arc Complex. In our favoured model (Figs. 6b, 6c), northward subduction beneath the Kara- koram margin gave rise to a mid-Late Cretaceous Japan Sea-type marginal basin along the Shyok suture, which then closed (possibly in response to collision with the oceanic Dras arc). An Early Tertiary Andean-type continental margin then

O + + 4- + + 4. Karakocam 4- ÷ ÷ + -P 4-

• • A

CLOSING I~OTETHYS Shyok ocean

• ~ i l g i t g n e i 6 ~ . . . . : , : . : . : . . = . . . T r e m a _ l . ; U l ~ a : : :-:: ?..-(. : .,...~ v v ~, " " " ' . ' . ' . . ~ . . . . . . : . - . . . . ' . - . ~:.

' v

CLOSING NEOTETHYS

80Ma

b + + Karakoram + + + ÷ ÷ ÷ 4- 4- 4-

Shyok marginal baskl

K E Y

~ F o r e a r c basin

I I O c e a n i c - ~ S u b d u c t i o n zone

I n d i a n p a s s i v e m a r g i n

C + + + Karakoram + + ~" + + + ÷ + + + -p

+ ÷ + -+ + + + Kohistan

K a r a k o r a m ( E u r a s i a n ) m a r g i n

T r a n s - H i m a l a y a ( L a d a k h B l o c k )

Kohistan-Dras a r c c o m p l e x

Accreted oceanic a r c

k r a u l - ~ . + + "4" +

CLOSING NEOTE'EHYS

CLOSING NEOTETHY$

8 0 M s 5 5 M s - - - - - - - - -

Fig. 6: Plate tectonic sketches showing alternative possible settings of the Dras arc Complex in the Late Cretaceous. (a) As the forearc of an arc within the Trans-Himalaya partly founded on continental crust that existed as a microcontinent within Neotethys. (b) As an oceanic arc, separate from an Andean-type active margin in the Trans-Himalaya and with the Shyok Suture to the north as a Cretaceous Japan Sea-type marginal basin. (c) The inferred continuation of (a) into the Early Tertiary. The main reasons for favouring (b) and (c) are strong compositional contrast between the Dras arc volcaniclastics and coeval Trans-Himalayan forearc sediments, and correlation with the oceanic Kohistan arc to the west.

A. Robertson, P. Degnan / Sedimentary Geology 92 (1994) 117-145 127

developed as northward subduction of Tethys continued, until collision with India by Early Eocene time (50 Ma). In the second part of the paper we now go on to describe and interpret the facies and petrography of individual units of the Dras arc Complex which allows reconstruction as an oceanic arc-forearc assemblage.

4. Lithofacies of the Dras 1 Sub-unit

The main lithofacies, as exposed in the Suru Valley, are as follows: Extrusives. Pillowed and massive flows, mainly basalt and basaltic andesite, make up a subordinate volume of the succession exposed in the Suru Valley (Fig. 7, log

L O G 1

~.,,~ . . . c--r* . ~ . ' . . 1 =

i ::!. :.:: i:~..:.:.:: . . . . . . . . . . . . . ~ t t

' - . . . . . . ¢ t ¢ 1 , , ,

L O G 2

? J s

(a)

~ n l l m

h

LOG 3

;ii!!i[i:i il " : ~ ° ®': : .

, . ' • - . . . - . ' . " ,

-R-- • w , . . . . . , . . . . . . . : . , . ' :1.

..:. ii..:::l.. :. :1 i

K U N O O R E R.

LOG 4

/ I I I I / ili:i: i:iJ / ; :: I " 1 "

L O G 5

I /x ~ A & IZ ~ ~ ~

i I . . . . . . .

I . . . . . . . . . . . . . .

~"t . . . . . ~, ~

i,:.....::-...:...., t

ea I A , A o A ,~ .6o~

I . . . . . . . . . : . . : . = I.'.'..'.-:-:-' : : .

I:... : . : I . "." : : : :: t '" . - . : .~ . i . : . : ' . : . - ' - -:::::::::::::::::::::::::

....a-.?.: ?

N of K U N O O R E R. NE SURU V A L L E Y W SURU V A L L E Y

Fig. 7. Measured sedimentary logs of the Suru unit in the Suru Valley area. See Fig. 4 for locations. (b) Key to logs in (a).


~ C lasts up to x cm in size t B ~ uP to Y cm

Colours

Gr Green Gy Grey Pk Pink O Orange R Red W White

iFe

o

iV

?-)

I

Iron-rich

Graded

Parallel lamination

Convolute lamination

Burrows

Slumps

Water escape structures

Plant debris

MAIN KEY

I I t ! I !

I

o o

A A A ~ A A A A A

(b)

Limestone

Calcareous sediment

Siliceous sediment

Shale I ' i '

Siltstone and sandstone [ I Conglomerate

Limestone conglomerate I - 0

Fine breccia t 5 Metres

Coarse breccia 10

Rip-up clasts GRAIN-SIZE

Pillow lava

Lava flow [

Fig. 7. (continued).

Well exposed sequence

No exposure

I Beds measured >lm

Beds interpolated

Fine Medium

Coarse

5). Lithologies range from clinopyroxene-, plagioclase- and or hornblende-phyric lavas, to aphyric lavas, preserved at zeolite and/or greenschist metamorphic facies. Individual flows range from several metres up to tens of metres thick and are typically interbedded with volcaniclastics. Pillow lava interstices are commonly filled with spalled, green devitrified glass. Quartz veining, epidosites and disseminated pyrite are locally developed. Massive flows locally engulf red chert.

Pyroclastic breccias. These are well-cemented, coarse fragmental volcaniclastics, both pyroclastic (eruptive) and epiclastic (detrital), interbedded with subordinate extrusives, that are commonly very vesicular. Flow breccias in this unit range from metre- to tens-of-metres-thick, commonly comprising angular to sub-rounded, plastically deformed clasts (e.g. west of the Suru Valley; Fig. 7, log 5) of pale green to dark grey andesite, with hornblende-phyric and aphyric facies.

Epiclastic breccias. Up to hundreds of metre-

thick successions (e.g. west of the Suru Valley and north of Kunoore River) are dominated by crudely stratified, poorly sorted, monomict to polymict volcaniclastic breccias, with a gritty volcaniclastic matrix (Fig. 7, logs 1, 2; Figs. 8a, c, d). The coarsest varieties contain up to metre-sized lava blocks, intercalated with discontinuous, thin intercalations of finer-grained volcaniclastics and rare chert. Well-rounded, water-worn clasts are occasionally observed.

Epiclastic sandstones and siltstones. Successions in the northern Suru Valley (e.g. north of Kunoore River; Fig. 7, logs 1, 2) are dominated by up to metre-thick alternations of commonly graded, locally slumped, volcaniclastic sandstones, siltstones and siliceous and/or pyritic shales (Fig. 8b). Typical sandstones are parallel laminated and grade up into greenish-grey siliceous shale. Local debris flows contain scattered, sub-rounded volcanic and/or limestone clasts in a shaley matrix.


Fine-grained siliceous facies. This compr i ses thin i n t e rbeds of r ed and g reen rad io la r i tes , o f ten assoc ia ted with massive and p i l lowed flows, th in uni ts of b lack sha le i n t e r b e d d e d with volcaniclas-

tic sands and shales (e.g. no r th of K u n o o r e River; F ig 7, log 2), and c o m m o n in te rca la t ions of pa l e acidic tuff, usual ly reworked .

Limestones. A consp icuous f ea tu re o f the Suru

Fig. 8. Field photographs. (a) Epiclastic volcanic breccia; sub-angular to sub-rounded clasts of vesicular andesite with a gritty volcaniclastic matrix. (b) Thin-bedded turbiditic volcaniclastic siltstone with thin siliceous partings (pale). (c) Coarse volcanic talus with a gritty and hydrothermal carbonate matrix. (d) "Plastic" deformation of volcaniclastic siltstone and impure chert, resulting from shear induced by movement of flow above. (a-d) Fig. 7, log 5. (e) Well-layered mainly extrusives of the Dras 2 Sub-unit (south of Kunoore). (f) Detail of interlava hemipelagic carbonates; Dras 2 Sub-unit (south of Kunoore).


Fig. 9. Field photographs of limestones in the Suru unit, Kunoore River area. (a) Limestone (pale, right), directly north of the Kunoore River (see Fig. 5). (b) Steeply dipping debris flows (Fig. 7, log 1). (c) Limestone debris flow with angular clasts in a volcaniclastic matrix, Kunoore River (Fig. 7, log 2). (d) Hornfelsed, deformed marble debris flows within contact metamorphic aureole of plutonic intrusive, northwest of Kunoore (eastern side of Suru Valley; Fig. 7, log 4). (e) Nindam Formation near Bodhkhabu (Fig. 5b). (f) Thin-bedded volcaniclastic turbidites with tuffaceous partings (Bodhkhabu section, Fig. 5b).

Val ley is the presence , locally, of large masses of La te Cre t aceous l imes tones (Reube r , 1987), which pr ior to this s tudy could be cons ide red e i ther as

d e t a c h e d blocks, in-si tu shelf l imes tone , or rede- pos i t ed l imes tones (Fig. 9a). The larges t (kilo- met re - s ized) exposures of sha l low-water ca rbon-


ates are located south and west of Somau, on the high Somau-Dras ridge, where the successions are well stratified, gently dipping and relatively undeformed. These units are assumed to comprise mainly in-situ shallow-water limestones (Reuber, 1989). During this study several of the more accessible successions were logged, particularly where well exposed in the Kunoore River (Fig. 7, log 3) and in cliffs just to the north (terminating against intrusives; Fig. 7, log 2).

The well exposed succession in the Kunoore River (Fig. 7, log 3) consists of amalgamated alternations of clast-supported limestone conglomerates (with sutured clasts), recrystallised massive limestone, bioclastic limestone, cherty limestone and soft grey siliceous and volcaniclastic siltstone (Figs. 9b, 9c). Successions exposed along strike on the cliffs above are more recrystallised and include alternations of coarse, partly silicified calcarenite (beds 15-40 cm thick), gravel- and pebble-grade conglomerates, with mainly sub-angular to locally well-rounded clasts. There are also fine-, medium- and coarse-grained volcaniclastic sandstones, showing parallel and convolute lamination. Locally folded successions, ca. 1 km to the northwest, consist of silicified marble, interbedded with fine-grained, black, pyritic volcaniclastic siltstone, cut by small syn-sedimentary faults. A contact metamorphosed succession ca. 4 km further north (Fig. 7, log 4; Fig. 9d) is made up of alternations of silicified volcaniclastics (hornfels), with thin (less than 14 cm) marble beds, mainly debris flows, with volcaniclastic and rare sub-rounded marble clasts (up to 15 cm long) in a matrix of pale vesicular silt. The marble beds include flattened iron-rich concretions, up to 3 cm in diameter.

Our observations show that: (1) limestone successions are wholly interbedded with arc-related volcanics and sediments and are not detached blocks; (2) redeposited limestones, epiclastics and marls are interbedded; (3) matrix-supported conglomerates dominate, with subordinate clast-supported conglomerates and common medium- to thin-bedded calciturbidites. None of the units studied are in-situ shallow-water carbonates, although these are assumed to be present in the

large outcrops west of the Suru River Valley (e.g. Somau ridge).

4.1. Petrography of the Dras 1 Sub-unit

The original lithology is commonly obscured by strong recrystallisation (e.g. formation of actino- lite schists by contact metamorphism). However, the Dras 1 Sub-unit (as redefined above) includes the following microfacies. (1) Lithic tuff: this is dominated by andesite lithoclasts with phenocrysts of angular, broken plagioclase, hornblende and augite in an altered glassy matrix, with secondary polycrystalline quartz. (2) Vitric tuff: this has volcanic glass altered to microcrystalline quartz, with secondary calcite and pyrite. (3) Epiclastic sandstones: these have variable admixtures of recrystallised acidic extrusives, basalt (with feldspar microphenocrysts), phenocrysts of pyroxene, altered feldspar and quartz, in a devit- rifled glassy matrix, with chlorite, epidote and carbonate. The fine-grained matrix contains small muscovite laths. Volcaniclastics interbedded with limestone (in the Kunoore River) contain highly recrystallised carbonate in a mainly recrystallised calcareous and volcaniclastic matrix, with scattered augite phenocrysts. (4) Acidic tuff: this has quartz and plagioclase phenocrysts in a heavily recrystallised mesostasis. (5) Mudstone: this comprises red volcaniclastic mudstone with recrystallised radiolarians and epidotised lava clasts. (6) Chert: this is present as microcrystalline silica with very poorly preserved radiolarians. (7) Lime- stones: entirely recrystallised.

4.2. Interpretation of the Dras 1 Sub-unit: arc interior and flanks

Arc core areas are assumed to include those cut by contemporaneous arc plutonics (e.g. pre- Dras 2 granodiorite). These units are relatively thin (less than 2 kin) and include volcaniclastics, subordinate andesitic extrusives, volcanogenic muds, tufts, and both in-situ and redeposited shallow-water carbonates. Once arc edifices reached sea-level, extensive erosion took place, leaving pedestals that were then capped or fringed


by carbonate platforms. The largest preserved Late Cretaceous carbonate platforms were located west of the Suru Valley and allochems were shed eastwards, together with volcaniclastic sediments, giving rise to turbidites and debris flows, as logged east of the Suru River Valley. The background sediment, where locally exposed, is siliceous and radiolarian-rich. Sediment instabil- ity in the arc core is marked syn-tectonic faulting, slumping and gravity redeposition. Individual volcanic centres cannot be reconstructed easily, in view of the large number of cross-cutting intrusives. The very thick, nearly massive Stang and Dandaldung mountains in the south are interpreted as very proximal arc-margin units. These were dominated by very coarse, angular volcanic talus, intercalated with subordinate pillowed and massive flows, interpreted as flank eruptions. The talus is assumed to have been shed southwards from arc core areas and includes clasts from ophiolitic basement and contemporaneous arc plutonics. A marine setting is confirmed by radiolarian background sediments.

5. Lithofacies of the Dras 2 Sub-unit

The type section of the Dras 2 Sub-unit, exposed north of the Kunoore River (Figs. 4, 8e), comprises a lower unit of intermediate composition extrusives, coarse volcaniclastics (in units up to 20 m thick) and subordinate red mudstones. This is overlain by a rhythmically layered succession of acidic extrusives, sills and minor sediments (Fig. 5e). Individual acidic lava flows range from 1 to 30 m thick, often with well developed flow-banding. The presence of characteristic de- vitrification textures indicates that the flows were originally mainly glassy. Thin sections reveal that the rhyolites consist mainly of microcrystalline quartz, with rare, altered small feldspar crystals. Some samples are partly replaced by calcite. The flows thin towards the southwest, individually and en masse. The succession is also cut by a number of sills up to tens of metres thick. The margins of several individual sills are packed with thermally altered xenoliths similar to the subjacent extrusives and sediments.

The lava flows and rarer sills are fiitercalated with sedimentary units, mainly less than 1 m thick. In detail, reddish finely laminated ferruginous mudstones are interbedded with lenses and partings of siltstone, thin-bedded, fine- to medium-grained sandstones and dark muddy micritic limestones (Fig. 8f). All these lithologies are calcareous. The thicker-bedded sandstones (up to 1 m thick) show grading, parallel lamination, micro-cross-lamination and small mudstone rip-up clasts. Palaeocurrents indicate flow towards the northwest. The limestones often form amalgamated units up to 30 cm thick, composed of several layers, each individually showing small- scale ripple-cross-lamination, planar lamination, basal scours, subtle grading and small-scale bur- rowing.

Higher levels of the Dras 2 Sub-unit are exposed further east, to the north of the Stang Mountain (Fig. 4). These are mainly acidic lavas that are locally folded and thrust-imbricated towards the north or northwest. A small outcrop of the Dras 2 Sub-unit, mainly volcaniclastics, is also exposed further northeast on both sides of the Kunoore River (southwest of Sapi). This outcrop is overthrust by a Mesozoic volcano-sedimentary unit of the Karamba unit, associated with the Lamayuru Complex (Sutre, 1990, and unpublished data).

5.1. Petrography of the Dras 2 Sub-unit

The following microfacies were identified. (1) Volcaniclastic sandstones and siltstones: these comprise very poorly sorted grains of microcrystalline quartz (i.e. devitrified volcanic glass), angular shards of clear, unstrained volcanic quartz and altered plagioclase. (2) Carbonate packstones: these are dominated by sub-angular to sub- rounded carbonate grains (micritic limestone, algal, shell and echinoderm fragments), together with scattered sub-angular lithoclasts of altered basalt and andesite, and rare crystals of feldspar, biotite and ferromagnesian minerals. Other samples are more heterogeneous, with extrusives of acidic, intermediate and basic composition, altered feldspar crystals and quartz shards. Vari- able secondary replacement by calcite, chlorite


and/or microcrystalline quartz is seen. (3) Mi- critic limestones: these are finely laminated, argillaceous and/or ferruginous and contain a diverse fauna of Radiolaria (replaced by calcite) and rare planktonic Foraminifera (recrystallised). Some samples contain admixtures of volcanic quartz, feldspar, biotite and opaque minerals.

not from a dissected arc, as in Kohistan. During later collision (in the Eocene?), the Dras 2 succession, as exposed in the east of the area, was tilted and unconformably overlain by alluvial clastics of Miocene? age (Indus molasse), followed, in this area, by backthrusting, then deep erosion in the later Tertiary.

5.2. Interpretation o f the Dras 2 Sub-unit

Around 75 Ma the Dras 1 Sub-unit was inferred by Reuber (1981) to have undergone a collisional event possibly related to accretion to the Trans-Himalaya and/or Eurasian margin to the north. As a result, the Dras 1 succession was folded and deformed in the north, while southern areas were less affected. There is, however, no evidence of deep erosion or subaerial exposure at this stage. Volcaniclastic deposition and arc-type volcanism of intermediate composition resumed. There was then a marked change to eruption of glassy acidic flows, assumed to have been erupted from fissures and then rapidly ponded into a broad topographic depression, possibly fault-con- trolled. The acidic volcanics presumably reflect eruption after collision and crustal thickening had taken place. The interbedded sediments are relatively fine-grained calcareous and volcaniclastic turbidites. The clastic sediment was reworked from volcanics of similar composition to the interbedded flows. Extrusion took place in a relatively deep, open marine setting, as shown by the abundance of radiolarians. The presence of common shallow-water allochems indicates the presence of neighbouring carbonate build-ups that were possibly re-established after deformation of the Dras 1 Sub-unit. Terrigenous sediment is, however, conspicuously absent. The Dras 2 Sub- unit can be compared with the ca. 60 Ma Baraul Banda Slate Formation in Kohistan, which unconformably overlies ca. 78 Ma plutonics and comprises deep-water volcaniclastic turbidites, interpreted as distal forearc facies of an Andean- type active margin. However, the Dras 2 Sub-unit in Ladakh differs in the following respects: (1) volcanics are present throughout the succession not only at the top; (2) facies are generally much finer grained; (3) provenance was mainly volcanic,

6. Naktul unit: mainly proximal volcaniclastics

The structurally overlying, elongate Naktul unit, exposed to the east of the Suru unit (Fig. 2), in the type area is dominated by thick-bedded, coarse volcaniclastic sediments, with subordinate pillow lavas (e.g. Pushkum-Shergol road section; Fig. 5c). This unit was thrust-imbricated towards the north and northwest, related to the regional Late Tertiary backthrusting. Thrusts are commonly marked by strands of sheared serpentinite. In contrast to the Suru unit (e.g. Suru Valley), arc plutonics are absent from the Naktul unit, which has undergone mainly zeolite facies metamorphism. Reconnaissance of the southwest slopes of Naktul Mountain showed that, although superfi- cially massive, most of the unit is made up of crudely stratified coarse volcanic talus, with subordinate more stratified finer-grained volcaniclastics, mudstones and local redeposited limestones.

6.1. Lithostratigraphy of the Naktul unit

In this study two successions were examined in more detail: (1) higher parts of the succession along the Pushkum-Shergol road; (2) southern slopes of Naktul Mountain.

In the Pushkum-Shergol section (Fig. 5c), pillowed and locally massive lavas are interbedded with rare red (locally green) radiolarian chert. Above comes a well-stratified, folded, mainly thick-bedded volcanogenic succession. Thicker- and thinner-stratified units alternate on a scale of tens of metres. The conglomerates vary from matrix- to clast-supported (in beds up to 4 m thick) and are mainly polymict, with angular, red and green andesite, glassy basalt (up to 20 cm in size), pumice lithoclasts and volcaniclastic mudstone rip-up clasts (up to 8 cm long), in a green volcani-


clastic matrix. The volcaniclastic turbidites exhibit Bouma A-D divisions (and combinations), including well developed parallel lamination (Bouma, 1962). Inter-turbidite facies are mainly greenish-grey calcilutites. Some thick-bedded, coarse sandstones and fine rudites (up to 2.5 m thick) are virtually homogeneous, with rip-up clasts (3-5 cm in size), flame structures and partings of grey/green volcaniclastic mudstone. Thin- ner-bedded and finer-grained turbidites are commonly interbedded with muddy micritic limestones, up to 30 cm thick, containing Globotrun- cana.

Further east, near Bodhkhabu (Fig. 5b) reconnaissance of the exposed base of the succession (structurally underlying melange) revealed massive and pillowed basalt and andesite, lava breccia of a pillow disintegration type, volcaniclastic sandstones and shales.

6.2. Petrography of the Naktul unit

The following microfacies are recognised: (1) Volcaniclastic rudites. These contain variable mixtures of extrusives and shallow-water-derived carbonates. Some samples are mainly igneous, with basalt lithoclasts, volcanic quartz, pyroxene, zoned plagioclase and hornblende crystals, silicic grains and biomicrite, set in a fine-grained chloritic and carbonate-rich matrix. Other samples contain both volcaniclastic (volcaniclastic siltstone and sandstone, basalt, andesite and rhyolite) and limestone (coral, calcareous algae, echinoderms and shells), together with volcanically derived crystals (mainly quartz, pyroxene, hornblende and plagioclase). Grains range form angular, to sub-rounded and locally very well rounded. (2) Volcaniclastic sandstones. These typically contain both monocrystalline and polycrystalline quartz, plagioclase (often altered), hornblende, clinopyroxene, quartz, basalt, andesite, rhyolite and derived epiclastic lithoclasts, together with variable amounts of calcitic allochems (including calcareous algae, echinoid plates and micrite grains), within a chloritic matrix. Most grains are angular to sub-rounded.

(3) Volcaniclastic silt. This is composed of fine- grained volcaniclastic sediment, with common small angular quartz shards and calcite-replaced radiolarians. (4) Silty calcarenite. This is dominated by angular grains of micritic limestone and other carbonate allochems, altered basic, intermediate and acidic composition lavas (largely carbonated), volcanic quartz, other volcanically derived crystals and rare biotite and muscovite (unstrained).

6.3. Interpretation: proximal volcaniclastic apron

The Naktul unit is interpreted as a proximal forearc volcaniclastic apron derived by erosion of an oceanic volcanic arc, including basic, intermediate and acidic composition extrusives. Previ- ously formed volcaniclastic sediments were eroded and redeposited. The common clast-supported breccias and rudites are interpreted mainly as proximal volcaniclastic talus deposited largely by rock-fall processes. Locally abundant matrix- supported conglomerates are identified as debris flows and high-density turbidites. The volcaniclastic sandstones and siltstones were deposited as turbidites. Grey-green volcanogenic mudstones are hemi-pelagic sediments, while pale grey mudstones and silstones are largely reworked acidic tuffs. The localised, thick units of pillow lavas, interbedded with radiolarian chert lower in the succession, are envisaged as marginal pillow lava volcanoes that were bypassed and/or overlain by volcaniclastic gravity flows. Muddy limestones higher in the succession are mainly pelagic carbonates. Pale silicic layers are water-lain tufts, variably reworked. In addition, rare intercalations of redeposited shallow-water limestone and allochems scattered through many of the redeposited sediments were derived from shallow- water carbonate platforms within arc core areas, probably located within the Suru unit to the west or north. Similar, proximal arc apron facies have, for example, been described from the modern Southwest Pacific (e.g. Clift, 1994), the Mesozoic of Baja California (Busby-Spera, 1988) and the Sierra Nevada, northwest California (Xenophon- tos and Bond, 1978).


7. Nindam Formation: distal arc margin successions

Our study confirms that the Nindam-Forma- tion (Figs. 10, 12a) is a folded and locally faulted, but essentially intact succession, in agreement with Sutre (1990). The Nindam Formation is cer- tainly not a thrust-imbricated subduction/accretion complex, as suggested by Garzanti and Van Haver (1988; Fig. 5a). During this study, reconnaissance of the Nindam Formation was carried out south of the Indus River, between Khalsi and Mongyu and also further west, in the Bodhkhabu area. In addition, a complete succession was stud-

ied in more detail in the vicinity of the road between Lamayuru and Khalsi ("Lamayuru loops"; Fig. 10). The mid-part of the succession there, which shows informative lithological varia- tion, was logged as far as possible on a bed-by-bed basis (Fig. 11). This was supplemented by study of the more uniform lower part of the succession in the Lamayuru River and the Yapola River, while higher parts of the succession were briefly examined downstream in the Yapola River. The reader is referred to Sutre (1990) for further details of less accessible successions.

The lower unit in the "Lamayuru Loops" area (Fig. 5a) consists of a ca. 250 m-thick, monotonous

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succession of fissile, pale grey-weathering tufts, turbiditic siltstones and turbiditic sandstones (in beds less than 1 m thick), in a siliceous volcanogenic matrix. Plant debris is locally abundant.

Above this, logging of the ca. 750 m-thick middle unit in the "Lamayuru Loops" road section revealed the following features (Fig. 11): (1) cyclicity is present, marked by repeated intercalations of volcaniclastic turbidites and pelagic carbonates; (2) individual sandstone packages are commonly sharp-based, with the thickest-bedded sandstones near the base and thinner-bedded sandstones higher up (but with some exceptions); (3) channelised debris flows are present, mainly associated with thick-bedded, amalgamated turbiditic sandstones (up to 3 m thick); (4) repeated intervals of pink/grey pelagic carbonate are interbedded with mainly thin- to medium-bedded and relatively fine-grained volcaniclastic turbidites.

The upper unit, ca. 850 m thick (Fig. 5a) marks a return to pale, fissile, mainly tuffaceous sediment, volcaniclastic siltstones and commonly thin- to medium-bedded turbiditic volcaniclastic sandstone (not measured bed-by-bed).

The following main lithofacies are present (Figs. 5a, 10).

Tufts. Fissile siliceous tufts, in layers up to 0.6 m thick, are present throughout the succession and volumetrically dominate the lower and upper units.

Volcanogenic mudstones. Grey, finely laminated, locally bioturbated, mudstone is abundant, particularly in the lower and upper units. The shale is occasionally dark and organic-rich, espe- cially in the highest exposed levels of the succession, near Mongyu.

Votcanogenic siltstones. Thin-bedded, graded, plane and/or parallel-laminated siltstones are interbedded with volcaniclastic turbidites throughout the succession and are conspicuously interbedded with pelagic carbonates in the middle unit.

Volcanogenic sandstones. These range from thin-, medium- to thick-bedded (Fig. 12b). The thick-bedded sandstones are typically lenticular on scales of tens of metres along strike, while the

medium-bedded sandstones are laterally continuous on an outcrop scale. Turbidites show Bouma A-D divisions and combinations (Bouma, 1962). Some medium-bedded sandstones exhibit sharp tops as well as bases, probably due to bottom current reworking. Lingoid ripples are occasionally well developed. Intraformatiomal rip-up clasts, up to 4 cm in size, are present near the base of some of the thicker-bedded sandstones. Deep-water trace fossils (e.g. Planolites; Fig. 12c) and woody plant material are locally present. Palaeocurrent indicators (e.g. flutes and groves) are occasionally visible on the soles of some thick-bedded turbiditic sandstones. After restora- tion to horizontal (assuming cylindrical folding) and removal of an assumed tectonic rotation (about a vertical axis), Sutre (1990) inferred sub- axial palaeocurrents flowing from the southeast. However, the data base is inadequate to reach any firm conclusion.

Matrix-supported conglomerates. These are a minor component, mainly of the m/dd/e unit (Fig. 11). Individual horizons, up to 3.5 m thick, are mainly associated with the thickest-bedded sandstones, but occasionally occur as discrete units within thin-bedded facies. Debris flows are lenticular over several metres, to tens of metres along strike. Individual horizons vary compositionally. Occasional debris flows in the lower unit contain rounded clasts of micritic limestone, up to 10 cm in diameter, in a volcaniclastic matrix. However, most debris flows are in the middle unit. Several- metres-thick units there include sandstone slumps (up to 1.7 m long), intraformational shale clasts (up to 1 m long), fossiliferous micrite and calcarenite, sub-rounded basalt, well-rounded intrusive rock clasts (granite, diorite) and rare red chert in a coarse sandy matrix (Fig. 11). These debris flows are eroded down into the tops of underlying beds. Occasional thin debris flows (up to 40 cm thick) contain mainly intraformational sandstone clasts in a massive sandstone matrix and are interbedded with amalgamated sandstone units up to 8.5 m thick. The limestone clasts are often moderately to well-rounded and contain rudist fragments, gastropods, pelecypods, calcareous algae, echinoderm debris, benthic foramini- fers and sponge spicules.


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Fig. 12. Field photographs and photomicrographs of the Nindam Formation. (a) View of the Nindam Formation above the Yapola River; the succession is folded and faulted, but otherwise intact. (b) Thin-bedded volcaniclastic silts and pink pelagic carbonates; middle part of the Nindam Formation, "Lamayuru Loops" succession. (c) Deep-water trace fossil assemblage on base of turbiditic volcaniclastic sandstone, Yapola River. (d) Polymict debris flow within thin-bedded volcaniclastic turbidites, middle unit, Yapota River, north of Wanla. (e-f) Volcaniclastic sandstones, dominated by angular to sub-rounded grains of mainly aphyric basaltic andesite; "Lamayuru Loops" road section (see Fig. 10).

Pelagic carbonates. These are thin-bedded, in units ranging up to tens of metres thick and are conspicuous, mainly in the middle unit (Fig. 11).

Individual sediment types range from thin-bedded pink pelagic carbonate to more varieoloured, shaley facies.


Calcareous mudstones. Pink to red calcareous mudstone and shale are interbedded with pelagic carbonates and volcaniclastics, mainly in the middle unit.

Chert. Bedded chert and cherty shale are mi- nor components, exposed mainly near the base of the succession.

Reconnaissance of the succession further west, near Bodhkhabu, revealed the following main lithologies (Fig. 5b). Turbiditic volcaniclastic sandstones, thin-, medium- and thick-bedded, in tens- of-metre thick units. Thicker, coarser-grained sandstones (up to 5 m thick) contain abundant small sub-rounded rip-up clasts of red siliceous mudstone. Some of the sandstones are well-sorted and show evidence of bottom current reworking. Grey volcaniclastic mud.stones, up tens of metre- thick units and as thin interbeds and partings. Deep-water trace fossils include Palaeodictyon. Pink pelagic carbonates and calcareous mudstones occur as up to tens-of-metres-thick intercalations near the top of the succession. Thin interbedded units and partings of black shale with abundant well preserved plant debris are also present.

7.1. Petrography of the Nindam Formation

The volcaniclastic sandstones are relatively constant in composition throughout the successions studied in the type area. Typical medium- grained sandstones contain lithoclasts of basalt, devitrified basic glass, dolerite (variably altered), rhyolite, rare radiolarian chert and opaque ore grains (Figs. 12e, 12f). Rare lithoclasts include finely crystalline mica-schist and polycrystalline quartz, quartzose sandstones and siltstone, with very small muscovite laths. Crystals are mainly quartz (plutonic, volcanic and microcrystalline varieties), including volcanic shards, plagioclase (often zoned), clinopyroxene and biotite (occasionally strained). Carbonate allochems range from fresh, to recrystallised and include micritic pellets, calcareous algal grains and benthic Fora- minifera. The matrix comprises abundant fine- grained quartz, biotite, small lithoclasts and fine silicic tuff.

Rare debris flows, with polymict clasts, were

found to comprise mainly glassy and chloritised basalt, recrystallised basalt (hornfels), volcaniclastic siltstone (with small volcanic grains), micrite, echinoderms, benthic Foraminifera, quartz (with resorbed margins), biotite and intraformational shale clasts (Fig. 12d). The plutonic clasts are mainly granodiorite and diorite, with pgroxene, muscovite and epidote. Plutonic grains are sheared and partly recrystallised.

Reconnaissance of the Nindam Formation in the Bodhkhabu section further west (Figs. 2, 5b) revealed a wide range of sandstone compositions. Sandstones lower in the succession are commonly arkosic, with zoned, altered plagioclase and rare orthoclase, hornblende, biotite and muscovite (unstrained). Minor components include basalt lithoclasts (with clinopyroxene phenocrysts), palagonite, chloritised basic extrusives, volcanic quartz crystals, altered rhyolitic and volcanically derived siltstone lithoclasts. The arkosic sandstones range from poorly sorted (in debris flows), to well sorted (in turbidites), with highly angular grains set in a fine-grained acidic (tuffaceous) matrix. Lithoclastic sandstones occurring throughout the succession are more polymict, with variable mixtures of basic/intermediate and acidic composition extrusives, reworked volcaniclastics, shallow-water limestones, abundant highly altered and variably recrystallised grains of mainly silicic extrusives and perthitic feldspar of intrusive igneous origin. Lithic sandstones are poorly sorted, but also contain sub-angular to moderately well-rounded grains.

Interbedded pelagic sediments, mainly towards the top of the succession, range from pink nearly homogeneous pelagic carbonates, with Globo- truncana, to calcareous radiolarian mudstones, with radiolarian tests variably preserved as microcrystalline quartz, calcite and chlorite. Common fine lamination and sorting of planktic fossils indicates accumulation in a current-active setting.

Z2. Interpretation of the Nindam Formation: distal forearc unit

The Nindam Formation is reconstructed as the distal part of an oceanic forearc apron that accumulated during the Late Cretaceous (Cenoma-


nian-Maastrichtian) in deep water, mainly as volcaniclastic turbidites, tuffaceous sediments and minor debris flows. The inferred setting is similar to the modern Barbados forearc apron (Sigurds- son et al., 1980). The volcaniclastic turbidites and debris flows are compositionally similar to those of the higher stratigraphic levels of the Naktul unit, although on average they are thinner-bedded and finer-grained. Much of this sediment is inferred to have by-bassed marginal slope apron areas to accumulate in deep water above the carbonate compensation depth (CCD). The arc first reached sea-level in pre-Aptian time, as recorded by reworked Aptian-Albian-aged shallow-water carbonate clasts. Large volumes of volcaniclastic sands were then redeposited into deep water during Cenomanian-Santonian time, probably corresponding to the time of maximum ele- vation of the arc above sea-level. This phase would be represented by deposition of the middle unit in the "Lamayuru Loops" succession, in which pelagic carbonate intercalations are nu- merous. Arc intrusions (e. g. granodiorite) must have risen, cooled and been unroofed by Albian time (ca. 100 Ma), providing rounded, to suban- gular, plutonic clasts, as found in some channelised debris flows. This is in keeping with the evidence from the Suru unit in the west that some arc plutonics are transgressed by the Early Tertiary? Dras 2 Sub-unit.

The large-scale, cyclical alternations of mainly volcaniclastic turbidites and pelagic carbonates can be related to relative changes in sea-level (volcano-tectonic and/or eustatic). During relative sea-level highs, the arc interior would have been flooded, reducing terrigenous run-off and favouring deep-water pelagic carbonate accumulation. During relative sea-level lows, the arc interior was then exposed to erosion and large volumes of votcaniclastic turbidites and minor debris flows, including plutonic clasts, accumulated on the adjoining apron. Dating is inadequate to infer the relative roles of local arc volcano/tectonic events, as opposed to global eustatic sea-level changes. However, the sharp bases of many of the thick volcaniclastic intervals could suggest that relative sea-level fell abruptly, stimulating turbiditic influx and that this was followed by slower

relative sea-level rise, marked by gradual de- crease in average bed-thickness and grain-size.

Later, during Campanian-Maastrichtian (upper unit), volcaniclastic sand input decreased. Continuing silicic tuff deposition possibly masked continued pelagic carbonate production during this time. Ocean arc volcanism, recorded by the Nindam Formation, effectively ended by the Pa- leocene, followed, in turn, by deep-water volcanogenic sedimentation, shallowing upwards with nummulitic deposition in the Early Eocene, then emergence in response to final collision of India and Eurasia in the Late Eocene.

8. Dras arc Complex units in the adjacent melanges

Lithologies of the Dras arc Complex are also structurally intercalated with adjacent melange units (Robertson and Degnan, in prep.). The Pushkum Melange (Fig. 2) structurally underlies the northwest margin of the Naktul unit and includes thrust slices and blocks of pillow basalt, depositionally overlain by thin radiolarites, then volcaniclastic turbidites. Also present are large disrupted sheets of serpentinite, cut by swarms of diabase dykes, similar to those within the ophiolitic basement of the Suru and Naktul units. Further east, the Mongyu Melange, north of the Nindam Formation (Fig. 10) includes detached blocks of pillow basalt, basaltic andesite, epiclastic and pyroclastic volcanic breccias, volcaniclastic siltstones, sandstones and conglomerates, also sheets of serpentinite, again cut by isolated diabase dykes. To the south, the Urtsi Melange (Fig. 10) includes disrupted thrust slices of arc-type pillow basalt (Sutre, 1990), lava breccias and volcaniclastic sandstones, with redeposited shallow- water carbonates. The Shergol Melange to the south of the Naktul unit (Fig. 4) includes serpentinite, again cut by diabase dykes. The arc-related units in melanges both north and south of the Dras arc Complex are interpreted as fragments of arc core (Mongyu Melange) and arc margin (Pushkum Melange) units that survived collision of the oceanic arc with the Ladakh Block (Trans-


Himalaya) to the north and with the Indian passive margin to the south.

9. D i scus s ion

As shown in Fig. 6, the Dras arc Complex could be interpreted, a priori, either as a continental margin arc or as an oceanic arc. This study has not revealed significant facies, or provenance differences between the Suru unit, the Naktul unit and the Nindam Formation, other than ex- plicable by proximal to distal trends, and thus we favour an origin as a single tectonic unit, rather than as composite, continental margin and oceanic units (cf. Sutre, 1990).

The Nindam Formation differs from the inferred Andean-type forearc succession in the Trans-Himalaya (Ladakh Block) to the north in important respects. (1) The two units are separated by tectonic melange interpreted as a suture (Robertson and Degnan, in prep.); (2) The Al- bian-Aptian Khalsi Limestones, part of the inferred Trans-Himalaya forearc succession (Van Haver, 1984; Sutre, 1990; our unpublished data) locally contain sandstones dominated by metamorphic quartz and other metamorphic minerals (e.g. muscovite), that are effectively absent from the approximately coeval Nindam Formation. (3) Terrigenous material (e.g. metamorphic quartz, schist) is not present to any significant extent in the Nindam Formation. Sutre's (1990) main evidence for a terrigenous provenance was rare metamorphic quartz, as seen in thin sections. However, similar polycrystalline quartz has formed within the Dras arc Complex by contact metamorphism, hydrothermal metamorphism and/or burial diagenesis. Indeed, petrographic work shows that much of the polycrystalline quartz resulted from recrystallisation of rhyolite. (4) The Upper Cretaceous-Lower Tertiary (Paleocene) Trans-Himalaya forearc succession overlying the Khalsi Limestones north of the tectonic melange (Gres Vert de Tar of Van Haver, 1984) includes thick channelised conglomerates with very abundant well-rounded granitic and rhyolitic clasts (e.g. near the confluence of the Yapola and Indus rivers and near Khalsi). By contrast, coeval suc-

cessions of the Nindam Formation (and above it) are deep-water hemipelagic and volcanogenic sediments of quite different composition (e.g. in the Manlung succession; Fig. 10). (5) If the Nin- dam Formation was derived from the Trans- Himalaya to the north, a very thick pile of basic- intermediate extrusives should be present to the north (overlying the Khalsi Limestone), but this is not observed. (6) The Cretaceous/Paleocene Trans-Himalaya forearc succession (Khalsi Lime- stone and overlying units) can be correlated with the Xigaze Group in Tibet, which also contains much acidic plutonic and terrigenous debris, unlike the Nindam Formation. Sutre (1990) suggests that the source of rounded shallow-water limestone clasts in the Nindam Formation was the Aptian-Albian Khalsi Limestone, which accumulated along the southern margin of the Trans- Himalaya unit. However, similar limestones are also present as coarse debris flows in the Suru unit in the west of the Dras arc Complex. We assume that similar shallow-water carbonates formed in the Upper Cretaceous both within the oceanic Dras, arc Complex and bordering the Trans-Himalayan continental margin arc.

In an oceanic setting, the Nindam Formation could in principle represent either a backarc apron, or a forearc apron, depending on subduction polarity. In the area between Namika-La and Khalsi (Fig 2), the proximal Naktul unit lies between the Trans-Himalayan batholiths and the more distal Nindam Formation. Also, slices of arc-type extrusives and volcaniclastics are present in the Mongyu Melange, north of the Nindam Formation, further east (east of Khalsi). Allowing for the effects of Late Cretaceous, Mid and Late Tertiary thrusting, it is probable that the Nindam Formation was always the most southerly unit. Thus, assuming northward suduction, a south-fac- ing forearc setting for the Nindam Formation is likely.

The Dras arc Complex is correlated with the oceanic Kohistan arc to the west. The Kohistan arc had been accreted to the Karakoram margin to the north by ca. 80 Ma, while in Ladakh, the Dras 1 arc unit was deformed at ca. 79 Ma, probably in response to collision with the Trans- Himalaya and/or Karakoram margin to the north.

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However, further east the Nindam Formation shows no evidence of deformation until Eocene collision with India. A possible explanation is that collision was diachronous. The arc complex may well have converged obliquely on the active margin to the north (Fig. 6b). Only the western part of the arc (Kohistan/W. Ladakh) collided force- fully at ca. 80 Ma, while the eastern extension of the arc remained undeformed as an inactive ridge within the forearc area during the Early Tertiary (Fig. 6c). This would also explain the absence of Early Tertiary Andean-arc plutonics, Dras-2 type acidic volcanics and proximal acidic forearc clastics from the Naktul unit and the Nindam Forma- tion.

Finally, the present outcrop pattern of the Dras arc Complex in Ladakh reflects a combina- tion of originally palaeogeographic, collisional and post-collisional deformation history. The large outcrop in the west (Suru Valley area) could relate to the palaeogeography of the Indian passive margin. A margin irregularity could have provided space necessary to preserve the western part of the Dras arc Complex during collision, while, further east, the Dras arc Complex was reduced to relatively narrow thrust slices and the inferred parent arc to the north was almost entirely destroyed.

10. Conclusions

The Dras arc Complex in Ladakh is interpreted as surviving remnants of a mid-Late Cre- taceous oceanic volcanic arc and forearc to the south. (Fig. 13). The Dras arc Complex formed part of a larger arc assemblage, including the Kohistan arc. In Ladakh, the Dras arc Complex formed on partly dissected Upper Jurassic? Tethyan oceanic crust. The arc complex is inferred to have comprised an arc interior (Suru unit) preserved in the west, a proximal forearc apron (Naktul unit) exposed further east, and a distal deep-water volcaniclastic apron (Nindam Formation) exposed in the centre and east of the area. By pre-Aptian time, arc edifices had reached sea-level, allowing fringing carbonate platforms to develop (mainly in the Aptian-Albian). During

the Cenomanian-Turonian, arc volcanoes were partly emergent, locally exposing plutonics to erosion (e.g. diorite, granodiorite). Large volumes of channelised volcaniclastic turbidites and debris flows were shed into deep-water forming a thick apron. Times of relative sea-level lows were per- haps marked by increased terrigenous run-off and erosion of marginal carbonate platforms, while mainly pelagic carbonate accumulated on the deep-water apron during times of relative sea- level high.

The arc interior exposed in the west (Suru unit) was accreted to the Trans-Himalaya/ Eurasian margin to the north in the Late Creta- ceous (ca. 79 Ma) and then unconformably overlain, along an Andean-type margin, by Pale- ocene? mainly acidic volcanics and fine-grained volcaniclastic turbidites containing radiolarians (Dras 2). By contrast, further east, there is no evidence that the forearc, represented by the Nindam Formation, was deformed until the Early Eocene (ca. 50 Ma). Oblique collision in the Late Cretaceous (ca. 80 Ma) is likely to have left the Nindam Formation as an offshore high within the Early Tertiary Trans-Himalayan fore-arc, until collision with India in the Early Eocene (ca. 50 Ma). The entire Dras arc Complex was then uplifted, partly eroded and overlain by coarse clastics derived from the Trans-Himalaya Batho- lith Terrane to the north. The Dras arc Complex subsequently underwent later Tertiary backthrusting and deep erosion.

Acknowledgements

We thank Mike Searle for introducing us to the geology of the Indus Suture Zone. This work was supported by an expedition grant from the Royal Society (1991) and the Carnegie Trust for the Scottish Universities (1993). Ian Sharp is thanked for assistance during the second field season. Elizabeth Bull helped with drafting and Yvonne Cooper with photography. Dr. Eric Sutre kindly provided a copy of his Ph.D. thesis (volume 1) soon after publication. Helpful comments on the manuscript were received from M.P. Searle and two anonymous referees.


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