doi:10.1144/GSL.SP.1993.074.01.04 1993; v. 74; p. 39-52 Geological Society, London, Special Publications

 Maurizio Gaetani, Flavio Jadoul, Elisabetta Erba and Eduardo Garzanti  

Karakoram: age constraints from sedimentary rocksJurassic and Cretaceous orogenic events in the North 

Geological Society, London, Special Publications

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London © 1993 Geological Society of

Jurassic and Cretaceous orogenic events in the North Karakoram: age constraints from sedimentary rocks

M A U R I Z I O G A E T A N I , F L A V I O J A D O U L , E L I S A B E T T A E R B A &

E D U A R D O G A R Z A N T I

Dipartimento di Scienze della Terra, Project Ev-K2 CEE, Via Mangiagalli 34, 20133

Milano, Italy

Abstract: At least two major episodes of tectonic deformation are recorded in the Mesozoic sedimentary succession of the North Karakoram. The older event took place between the Liassic and the early part of the Mid-Jurassic, as documented by tens to hundreds of metres thickness of dark grey litharenites (Ashtigar Fm.). The latter are sharply overlain by 35 m to 150m thick red sandstones, interbedded with shales and siltstones (Yashkuk Fro.). Terrigenous bodies increase in thickness from west to east, and from north to south. Petrographic composition indicates the orogenic character of these quartzo-lithic sand- stones, characterized by mafic volcanic and serpentinite detritus, being replaced upwards by sedimentary and subordinate metasedimentary clasts. The Ashtigar Fro. may have been deposited in a collisional basin close to a newly-formed suture belt. Contribution from allochthonous oceanic rocks was significant in the first stages of collision, whereas detritus was subsequently derived mainly from uplifted sedimentary successions. Terrigenous units were conformably transgressed by shallow-water limestones in the Aalenian, and later on marine conditions persisted until the earliest Cretaceous.

The second event of tectonic deformation occurred between the earliest and the Late Cretaceous, when the fan delta Tupop Conglomerate, mostly derived from uplifted carbonate and clastic rocks, sealed the strongly folded and faulted underlying succession. Mid-Cretaceous granitoid plutons are also widespread in the North Karakoram. This orogenic event is interpreted as related to the final welding of the Kohistan arc and Karakoram microplate to Asia.

The highly irregular topography of the Tupop Conglomerate is onlapped unconformably by reddish to grey nodular limestones, some hundred metres thick and containing phosphatized inoceramids and nannofossils of Campanian age (Darband Fm.). These are the youngest marine sediments detected in the Karakoram to date.

The aim of the present paper is to provide new evidence to constrain the age of the early deformations that affected the North Kara- koram during the Mesozoic, before final welding of the Indian Plate against Eurasia, and thus before the Karakoram Range was built into its present shape by Himalayan collision tectonics. The sedimentary succession has been studied in a narrow belt extending from the head of the Chapursan valley in the west to the Shimshal pass in the east (Fig. 1). Since this belt (Sost Uni t of Gaetani et al. 1990a) has been disrupted into several tectonic sheets, stratigraphic analysis is not always straightforward (Fig. 2). For details about this mainly Cenozoic complex structural evolution, the reader is referred to Zanchi (this volume).

Stratigraphic knowledge of the area was poor before our 1986 and 1991 expeditions. Only Desio & Martina (1972) provided some data for the area around Sost, and Casnedi & Nicora (1985) reported a few Palaeozoic fossils from the Shimshal valley. Gaetani et al. (1990a) pre-

sented the first stratigraphic scheme for the Chapursan area.

The Jurassic succession

Rocks of Jurassic age crop out both in middle Chapursan, from the Kundil valley to the Yash- kuk area, and in the Shimshal area, from Zardinarbin to Parien. A thick and complete terrigenous succession crops out in most of the investigated region, whereas a reduced se- quence is confined to the northwest (Yashkuk Fro. of Gaetani et al. 1990a).

Lithostratigraphy

From base to top, the following units were recognized.

(1) Up to 700 m thick, massive dark grey limestones and thick-bedded peritidal dolomites (Aghil Group, Desio 1980; Gaetani et al. 1991). Metre-scale regressive cyclothems contain large megalodontids of Late Triassic affinity. Blocks

From TRELOAR, P.J. & SEARLE, M.P. (eds), Himalayan Tectonics Geological Society Special Publication, No. 74, pp. 39-52.

39

40 M. GAETANI ET AL.

,,' /

~, • ~.~.'..~ U Y ~ _)__k ~ ~ ~ ~". . " - • " VI 7 \

2_N __2__A

/

u .~-

I

O"l~ Ip I C ~ ~" \ ~ J

.,g ,,,,

a.

I I

g!

r - - - . )

1 \

) I

/

*,~ i !

q

; / I tl ;U/ / I

/ ~ _ ~

packed with Lithiotis occur in the scree, but could not be positioned within the succession. The Aghil Group interfingers towards the south- east with breccias and cherty limestones de- posited in deeper waters.

(2) Up to 70 m thick, thin-bedded, dark-grey biocalcarenites and biocalcirudites very rich in crinoids, interbedded upward with thin grey shales and platy laminated mudstones.

(3) Dark-grey splintery shales and marls, tens to hundreds of m-thick, occurring in the South Chapursan and Kundil valleys and Shimshal district (Zardgarbin to Parien). Distal turbiditic beds, 15 to 80 cm thick and displaying Tb-Te Bouma intervals, pass upwards into a more continuous shaly succession. Overall thickness is between 100 and 200 m in Chapursan (Fig. 3), and perhaps more in Shimshal, where greater thickness may be ascribed to tectonic rep- etitions. We propose here the term Ashtigar Formation for this unit (Fig. 4).

The type-section was studied at 4350 to 4450 m altitude, on the ridge southeast of the Ashtigar meadows. The lower boundary is marked by a sharp transition from the calcare- nites/calcirudites and dark laminated limestones of the topmost Aghil Fm. to black shales with intercalated lithoclastic calcarenites yielding volcanic debris. These 20 m thick basal Ashtigar shales are overlain by at least 70 m thick, dark grey marly shales with interbedded, fine-grained turbidites. The upper 20 m of the unit consist of alternating dark grey and greenish shales, with subordinate sandstones. The upper boundary corresponds to the erosional base of a 5 m thick interval of amalgamated conglomerates with abundant clasts of deformed Aghil carbonates and rare volcanic grains, followed by greenish pelites and greenish-reddish sandstones (Yash- kuk Fro.).

In North Chapursan, the Ashtigar Fm. is missing. It is apparently replaced by very fine- grained hybrid arenites and dark grey, well- bedded, bioclastic packstones/wackestones rich in crinoids, brachiopods and foraminifera. This interval (hereafter called unit 2A), which is about 40-50 m thick and arranged in thickening- upward sequences, has no counterparts in the southern successions, and its base is covered.

(4) Fine-grained, red and green sandstones and siltstones in decimetre-thick beds, com- monly intercalated within thin red to greenish- grey shales (Yashkuk Fm. of Gaetani et al. 1990a). Their base is locally marked by cal- careous breccias or intraformational con-

Fig. 1. Location map of the investigated area.

M E S O Z O I C H I S T O R Y OF T H E N O R T H K A R A K O R A M 41

g

e.d, . . , , ,~

E ,~

z ~ ~E

r4~

N

° ,,...~

° , . . .

e ~

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N .a ~ ' N

~ ' ~ •

,, e~"

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42 M. GAETANI ET AL.

Fig. 3. The two Jurassic sandstone units in the Ashtigar section. Ash, Ashtigar Fm. ; Yas, Yashkuk Fm.

glomerates, 3 to 6 m thick, which may erode the underlying Ashtigar Fm. Metre-scale lithic con- glomerate lenses with red matrix also occur in the Pamir-i-Tang area below Wir Sar (Zard-i- Gar according to Shipton 1938). Red sandstones are rare or absent altogether in the east, where the unit may reach a thickness of 150 m (Fig. 3), and become more abundant in the west where, in the northern belt of Jurassic outcrops from north of the Yashkuk glacier snout to the Karakoram Highway north of the Kilik-Hunza river confluence, they predominate but are reduced to a few tens of metres (Fig. 4).

(5) The calcareous succession overlying the terrigenous units can be subdivided into two parts, from bottom to top: (5A) dark grey, oolitic or oncolitic, subtidal bioturbated pack- stones (about 160m thick); (5B) dark grey, well-bedded mudstones and wackestones, lo- cally with chert nodules (at least 150 m thick; Reshit Fm. of Gaetani et al. 1990a).

Units 5A and 5B may be locally replaced by dark grey, thick-bedded, amalgamated pack- stones/wackestones with rare chert nodules (thickness up to 200 m).

Superposition of sub-units 5A and 5B is locally doubtful, because of extensive disruption by closely-spaced strike-slip faults. Also the correct stratigraphic position in this part of the

succession of anhydrite layers, stromatolitic mounds interbedded with grey-green shales, and Cayeuxia-bearing dark mudstones could not be ascertained due to faulting.

(6) The youngest carbonate unit, cropping out near the snout of the Yashkuk glacier, apparently consists of metre-scale megabreccias containing clasts of shallow-water carbonates, and calcarenites intercalated within thin beds of pinkish hemipelagic wackestones.

In summary, a complex carbonate platform (Aghil Group) was gradually drowned; the newly-formed basin received increasing ter- rigenous detritus, episodically transported by gravity flows (unit 3). At the flanks of the trough, shallow-water hybrid carbonates possibly con- tinued to be deposited (unit 2A). Later on, the terrigenous Yashkuk Fm. (unit 4) spread over the area, which was characterized by deltaic to marginal marine conditions. Gradually, the sea returned over the area and shallow-water car- bonates were deposited.

Biochronological data

Unit 1: Aghil Group. Biostratigraphic data are poor in the lower part of the unit. In the middle part, megalodontids and carbonate platform

MESOZOIC HISTORY OF THE NORTH KARAKORAM

NORTH SIDE RESHrr VALLEY ASHTIGAR

CHAPURSAN VALLEY (southern tectonic slice) VALLEY

43

nail" Watznaueria manivitae, =.xafithus magharensis

R E S H I T Fm.

\ \

lall" Hexafithus magharensis \ \

" O N C O I D A L

L I M E S T O N E S " / / I /

f

/ /

I /

. i . /

C A S H K U K Fm.

~lesoendothwa sp.

L I J - - J

a a

Ha~anTa sp. - . . . . . . . . . . . r

Lotharingius hauffii, Mitrolithus jansae Schizosphaem//a punctulata "Spiriferina" sp.

<3

//

-T-Tn--

- - V -

=..-r~

F

L E G E N D

Brachiopods ~ Mud cracks

Ammonoids ~ Erosional surface

Corals / v , Ripple marks

Megalodont ids o ® Ooids

Pelecypods ~, "* Tepee structures

Echinoid, Crinoids ~ Stromatol i tes

Dasycladaceae ~ Burrowings

Calcareous Nannofossi ls (~ Oncoids

Foraminifera - - Chert laminations

Bedded l imestones - ~ ' - Cross laminations

Thick bedded l imestones . . . . Shales-marls

Calcarenites - rudites B Black shales

Conglomerates Siltstones

Fault, thrust v v v Evaporites

Fig. 4. Representative stratigraphic columns measured in the Jurassic units.

~UK Fm.

IAR Fm.

L A T F O R M

PE F A C I E S "

G H I L Gr .

i 1 0 0 m

0

faunas of Norian/Rhaet ian affinity have been recognized (Aulotortus sp., Ophtalmidium sp., Miliolipora sp., Thaumatoporella sp.). Since in

the Reshit side valley Lithiotis has been found in loose blocks, a Late Triassic-earliest Jurassic age is inferred.

44 M. GAETANI ET AL.

Unit 2. Bioclastic packstones/rudstones contain within the clasts a microfacies usually char- acterizing Upper Triassic carbonate platforms, including benthic foraminifera (Aulotortus sp., Galeanella sp., Ophtalmidium sp., Glomo- spirella sp., Nodosaridae and Miliolidae), Dasy- cladacean algae like Thaumatoporella, crinoids, bryozoans and a few Radiolaria.

Unit 2A. We found two intervals containing age-diagnostic fossils. The lower one consists of 6 to 7m thick marly limestones, marls and bioclastic packstones. Its base yielded a brachio- pod fauna with Zeilleria sp. and 'Spiriferina' sp., whereas in the marls 1.7 m above a nannoflora with Mitrolithus jansae (Wiegard), Lotharingius hauffii Gr/in & Zweili and Schizosphaerella punctulata Deflandre was found. The brachio- pod assemblage does not reach into the Toar- cian, because of the presence of 'Spiriferina'. Nannofossils point more precisely to a latest Pliensbachian-Early Toarcian age.

The upper interval, forming the topmost 15 to 20 m of the unit overlain by the Yashkuk Fm., is a regressive sequence characterized by bioclastic crinoidal packstones grading into oolitic bioclas- tic packstones enriched in quartz grains. It yielded foraminiferal assemblages including Haurania and Involutina. The latter apparently does not range above the Pliensbachian. A latest Pliensbachian age may thus be assigned to unit 2A.

Unit 3: Ashtigar Formation. Age data could not be obtained from this unit. Along the trail between Targir dur and Parien, some fragments of Trigonia-like bivalves have been extracted by L. Angiolini.

Unit 4: Yashkuk Formation. Age data could not be obtained where the unit is thicker. In the Yashkuk section a bioclastic packstone inter- calated in the middle part contains abundant Mesoendothyra sp., pointing to a Toarcian or early Mid-Jurassic age.

Unit 5A. In the Yashkuk section, oncolitic limestones include the foraminiferal fauna listed in Gaetani et al. (1990a, p. 59). Two new levels with nannofossils have been found (Fig. 4). The lower one, 150 m above the top of the Yashkuk Fm., contains Schizosphaerella punctulata and small Hexalithus magharensis Moshkowitz & Ehrlich, and could thus be assigned to the Late Aalenian. In the pelagic limestones above, a 10 m marly intercalation yielded Schizosphaer- ella punctulata, Hexalithus magharensis and small Watznaueria manivitae Bukry. Age: Ba-

jocian, because of the appearance of small Watznaueria.

Unit 5B: Reshit Fm. Some brachiopods (Burmirhynchia sp., a new genus of tererebra- tulid close to Monsardithyris) and a single speci- men of the ammonoid family Perisphinctidae, pointing to a Mid-Jurassic age, have been found. Nannoflora, with small Watznaueria manivitae, Cyclagelosphaera margarelii No61, Discor- habdus criotus Bown, Hexalithus magharensis confirm that suggestion.

Within the shales intercalated with the an- hydrite lenses, a nannoflora with Watznaueria cf. communis Reinhart has been recognized. The genus Watznaueria spans the interval Ba- jocian-Cretaceous.

Unit 6. Small Protopeneroplis sp., indicating a latest Jurassic-earliest Cretaceous age, are fairly common in resedimented beds. The presence of a nannoflora comprising Watznaueria barnesae (Black), Rucinolithus terebrodentarius Apple- gate et al., Nannoconus sp., and Braarudo- sphaera sp. in hemipelagic intervals points to the earliest Cretaceous. Orbitolinids have not been found in the Chapursan valley sedimentary rocks so far.

In summary, the Ashtigar Fm. can only be generally ascribed to the Early Jurassic, since it overlies the Aghil Fm. which may contain Lit- hiotis beds. The Yashkuk Fm. in contrast is fairly well bracketed between the latest Pliensbachian and the Late Aalenian. In Gaetani et al. (1990a, p. 58-59), the foraminiferal assemblages below and above the sandstones were considered as similar, and thus the unit was ascribed to the Pliensbachian; this conclusion is amended by new data.

Sandstone petrography

The Jurassic sandstones are litharenites (only a few samples with grain size around or below 150 ~m are feldspathic litharenites) with com- mon volcanic, sedimentary and metedimentary rock fragments. Main petrographic parameters (Dickinson 1970) of 15 fine to lower medium- grained analysed samples are comprised in the range Q 48 to 72; F 0 to 17; L 17 to 47. Secondary parameters are C/Q up to 21, P/F up to 89, V/L up to 89, S/L up to 62, M/L up to 27.

Ashtigar Formation. Very fine to lower medium- grained sandstones (detrital modes Q 55 _+ 5, F 9 _+ 4, L 35 + 4; Fig. 5) contain common radio- larian chert, chlorite schist, serpentine schist and basaltic rock fragments, strongly testifying to

MESOZOIC HISTORY OF THE NORTH KARAKORAM 45

erosion of an oceanic subduction complex (one eighth to one fourth of total detritus). Volcanic lithics are common (V/L 49 to 61), and comprise both 'basaltic' types, displaying plagioclase laths interspersed within a chloritized glassy ground- mass, and vitric to felsitic grains. A few particles contain both quartz and plagioclase phenocrysts embedded in glassy groundmass, and were de- rived from the products of felsic explosive activity. Quartzose terrigenous (siltstone to sandstone) rock fragments also invariably occur along with extrabasinal carbonate grains (do- lostone and limestone). The sedimentary to total lithic ratio (S/L 20 to 41, including chert grains) tends to increase upward; in some samples it may have been overestimated due to intra- basinal reworking, indicated by local finding of oversized serpentine schist-bearing quartzose silty clasts and by common occurrence of clayey intraclasts.

Detrital quartz is mainly monocrystalline (oc- casionally displaying straight extinction or rounded outlines), but polycrystalline grains, with mainly fine crystal size and irregular to stretched intercrystalline boundaries, are signifi- cant (C/Q 13 to 20, tending to increase with grain size). Feldspars are not abundant, with pre- dominating plagioclase grains (P/F 67 to 89). Among the alkali-feldspars, chessboard-albite and more rarely orthoclase and microcline occur. Phyllite to paragneiss rock fragments (M/L 8 to 24, including serpentine schist grains) are invariably present, suggesting that low-grade metasedimentary rocks were exposed in the source areas and supplied a significant amount of quartz and subordinately albite grains. Blue- green to brown tourmaline, zircon, rutile and muscovite were sporadically recorded among the accessories, along with single radiolarians with opaque rims.

Yashkuk Formation. Detrital modes of fine- grained sandstones are Q 59 + 8, F 6 + 6, L 35 + 10 (Fig. 5). Sandstones are moderately sorted and very fine to coarse grained. They contain insignificant amount of 'basaltic' and serpentine schist grains. Carbonate and subordi- nate terrigenous and radiolarite sedimentary lithics (S/L 43 to 70 in the lower part, passing upward to S/L 27 to 28; chert grains included) are instead more abundant, as well as phyllite to paragneiss rock fragments (M/L 6 to 27, with values over 50 reached in the Misgar area). The abundance of sedimentary lithic fragments is strongly controlled by grain size (r = 0.84; 1% sign. level), and the rare microconglomerate layers have calclithite compositions. Felsic rock fragments are as common as in the Ashtigar Fm.

O

v ~

F / ~ 50

Le

Fig. 5. Detrital modes of selected Jurassic sandstones (Yashkuk and Ashtigar Fms.), which plot in the 'recycled orogenic' field (Dickinson, 1985). Q quartz; F feldspars; Lc aphanite lithic fragments (carbonate grains and chert included). Polygons are one standard deviation each side of the mean. Abundance of chert, basalt, and serpentine schist fragments in the Ashtigar Fro. (circles) and predominant sedimentary- metasedimentary detritus in the Yashkuk Fm. (triangles) point to 'subduction complex' and 'collision orogen' provenance respectively (Dickinson & Suczek 1979).

but, due to absence of lathwork 'basaltic' grains, the volcanic lithic population is generally re- duced (V/L 24 to 50). Only at Yashkuk, where sedimentary lithics are lacking, rock fragments are mostly volcanic felsites (V/L 80 to 84).

Polycrystalline quartz is subordinate (C/Q 14 to 21 in samples coarser than 150 p~ and 3 to 9 in finer-grained samples). Detrital plagioclase is not very abundant (P/F < 60); alkali-feldspars include orthoclase and albite. Feldspar content tends to decrease from lower fine-grained samples (F 5 to 17) to sandstones coarser than 200 ~m (F less than 5). Euhedral zircon, blue tourmaline and muscovite were recorded.

Intrabasinal mudclasts (illite-chlorite rip-up clasts and caliche fragments) may be locally abundant (up to 20% of the rock). Marine bioclasts (echinoderms, bivalves, benthic fora- minifera, corals, bryozoans) sometimes occur in reddish layers. Single radiolarian tests were also recorded.

In the upper part of the Yashkuk Fm., where sandstones are invariably very fine-grained and interbedded with oolitic sublitharenites, hybrid oo-biocalcarenites, peloidal micrites, calcareous siltstones and subordinate stromatolitic bind- stones, the mineralogy of the extrabasinal frac- tion is more difficult to assess quantitatively.

46 M. GAETANI ET AL.

Source areas. The quartzo-lithic petrographic composition of the Jurassic terrigenous for- mations is typical of orogenic clastics. Detrital modes, plotted on a standard QFL triangle, fall in the middle of the 'recycled orogen' proven- ance field (Dickinson & Suczek 1979; Dickinson 1985). Mineralogical composition of the Ashti- gar Fm., characterized by common serpentine schist, basalt and chert lithic fragments, as well as by detritus from sedimentary, metasedi- mentary and felsic volcanic rocks, documents erosion of a tectonically-uplifted thrust belt in the proximity of an arc-trench system. The source area included a subduction complex composed of deformed ophiolites and other oceanic rocks, emplaced adjacent to a foreland basin that developed during the first stages of arc-continent collision.

The Toarcian-Aalenian Yashkuk Formation documents the structural evolution of the fore- land fold-thrust belt, with further uplift of the accretionary prism and extensive erosion of thrust sheets made of sedimentary and subordi- nately of low-grade metasedimentary rocks. Supply from ophiolitic melanges was strongly reduced. Neither uplifted basement block nor granitoid intrusive sources are ever indicated.

The Cretaceous succession

Information on the Cretaceous deformation event was obtained mainly in the Kundil and Borom side valley, in lower Chapursan (Fig. 2). In other areas the Tupop Conglomerate is

present as well, but sections are disrupted by fold-thrust and strike-slip deformation (Zanchi, this volume) and reliable stratigraphic data could not be obtained.

Tupop Conglomerate

Gaetani et al. (1990a) introduced the term Tupop Conglomerate for the lithic conglomer- ate which seals the underlying sedimentary sequence, including basaltic rocks of unknown age, with spectacular angular unconformity (Fig. 6a).

The Tupop Conglomerate occurs from the Ziarat of Babagundi in the west, to the heads of the Kundil and Borom valleys, and up to the Shimshal area in the east (Pamir-i-Tang), where it may be observed in the gully where the hut of Arbob Parien is built.

The Tupop Conglomerate lacks age- diagnostic fossils. Petrographically, terrigenous layers are invariably calclithites, containing pre- dominant sparitic carbonate rock fragments along with frequent sandstone, siltstone and shale clasts (Fig. 7). Also felsic volcanic lithics are common. Monocrystalline quartz occurs and commonly shows good roundness or em- bayments due to magmatic resorption, pointing respectively to polycyclic sedimentary and felsic volcanic provenance. Most calcareous lithic grains are extensively recrystallized, but oolitic or bio-intraclastic grainstones and biomicrites are sometimes recognizable. Calclithite layers yielding abundant reworked echinoderm frag- ments and Permian fusulinids are also found.

(a) (b)

Fig. 6. Stratigraphic relationships in the west side of the Upper Kundil valley, (a) The angular unconformity at the base of the Tupop Conglomerate, resting on shales of the Ashtigar Fm. Thickness of the Tupop bank is nearly 120 m. (b) Back-view of the same Tupop Conglomerate, onlapped by the Darband Fm.

MESOZOIC HISTORY OF THE NORTH KARAKORAM 47

Interpretation of depositional environments is problematic. Fluviatile, fining-upward fes- tooned bars were observed near the Ziarat of Babagundi (Chapursan valley); in the Borom valley, instead, red nodular unfossiliferous lime- stones are intercalated also in the basal part of the unit, below the up to 60m thick con- glomerate bank, possibly suggesting deposition in deep-marine environments. Very steep cross- bedding in the basal conglomerates (Borom valley) suggests a fan delta environment for this part of the unit.

In summary, the Tupop Fm. is a lithic con- glomerate derived from erosion of sedimentary and very low-grade metasedimentary source rocks ('recycled orogen' provenance of Dickin- son & Suczek 1979). Since detritus contains all of the lithologies characteristic of the underlying Permo-Mesozoic stratigraphic units, local sup- ply from the deformed and uplifted Karakorum sedimentary succession is indicated.

Fig. 7. Close-up view of the Tupop Conglomerate, Kundil valley, West side. Scale bar is 15 cm long.

Among the cobbles, grey micritic limestones with Dasycladacean algae, bivalves, gastropods or foraminifera, reddish fine-grained quartzose sandstones or rarer sedimentary litharenites, grey or reddish oncoidal limestones, dark-cherty limestones and calcareous microbreccias, and a few peritidal carbonates with stromatolites, fen- estrae or vadose pisoids, are recorded in this order of abundance. Composition of clasts com- pares well with the lithologies of the underlying Permian to Jurassic units. Thorough recrystalliz- ation of limestone clasts and well-developed cleavage in argillite rock fragments point to very low-grade metamorphic deformation in the source rocks.

Notable are: (a) lack of crystalline basement clasts, in contrast to the similar Urdok (Gaetani et al. 1990b, 1991) and Reshun Conglomerates (Pudsey et al. 1985); (b) lack of Orbitolina- bearing clasts, in contrast to the similar Reshun Conglomerate of the nearby Karambar valley (Casnedi 1980) and Chitral (Hayden 1915); (c) lack of pachyodont bivalve fragments, which are reported in Chitrai (Pudsey et al. 1985).

D a r b a n d Format ion

The Tupop Conglomerate is overlain with a spectacular onlap by red and grey nodular wackestones in amalgamated beds (Figs 6 & 8). For this unit we propose here the term Darband Formation, from the Darband tower at the end of the Kundil valley, where we first observed loose blocks of these Scaglia-like mudstones.

Bedding forms an angle of 10 ° to 15 ° to the underlying conglomerates (Fig. 9). Total thick- ness is more than 300m. At the top, these limestones are once more interbedded and over- lain by concordant lithic conglomerates with abundant red matrix.

The unit crops out only at the head of the Borom and Kundil valleys, possibly extending up to the head of some side valleys south of the Lupghar valley, which were not reached during our expeditions. Younger units are not exposed in the studied area.

Microfacies have been largely destroyed by pervasive deformation and slaty cleavage. Only phosphatized fragments of inoceramids and ghosts of possible globotruncanids were ob- served. Search for coccoliths was more fruitful, and two specimens were found in sample KK191b: Ceratolithoides verbekii Perch-Nielsen (spread in the Early and Middle Campanian, CC18 to CC21) and Calculites obscurus (De- flandre) (Campanian to Early Maastrichtian, CC17 to CC24).

The Tupop Conglomerate is thus bracketed between two units of earliest Cretaceous and Campanian age, being separated from them by angular unconformities.

48 M. GAETANI ET AL.

Fig. 8. Stratigraphic relationships in the east side of the Upper Kundil valley. Spectacular onlap of the Darband Fm. on vertical Jurassic limestones. The rock wall is about 400 m high. The normal fault seen in the centre of the photo is interpreted as syngenetic to deposition of the Darband Fm. (Zanchi, pers. comm.).

. . . . . . . . .

i ..... , ,or. ioo: i i i'I Fig. 9. Geometrical relationships between Jurassic and Cretaceous units at the head of the Kundil valley.

The Early Jurassic orogenic event

Early Jurassic turbiditic sandstones containing detritus from oceanic rocks are interpreted as derived from a subduction complex and de- posited in a collisional basin close to a suture belt. A nearby carbonate ramp was apparently not affected by this episode. Turbiditic deposits occur in the south and east, and the carbonate ramp in the north and northwest. In the

Toarcian-Aalenian, a deltaic to marginal ma- rine 'recycled orogenic' red sandstone unit pro- graded onto the whole area and sealed the previous palaeogeography. The unit is thicker where turbidites were previously deposited, and thinner and finer-grained where platform car- bonates persisted. Facies distribution and palaeocurrent directions suggest that detritus was derived from the southeast; later extensive fold-thrust and strike-slip deformation, how- ever, may have severely changed the original palaeogeography.

Comparisons with proximal areas

Shaksgam valley, East Karakoram and Aghil Range. Red conglomerates and sandstones (Marpo Fm.) have been recently described in the Shaksgam valley (Gaetani et al. 1990b, 1991). Benthic foraminifera suggest a late Mid- Jurassic age, thus younger than that of the Yashkuk Fm. Similar quartzo-lithic sandstones/ microconglomerates associated with anhydrites and mudstones occur in the Surukwat area (Gaetani et al. 1990b, 1991).

MESOZOIC HISTORY OF THE NORTH KARAKORAM 49

West Tibet. In the southwestern Lingzi-Tang area, De Terra (1932) reported red sandstones and conglomerates overlying fossiliferous No- rian 'limestones' and followed by grey-yellowish calcareous marls containing bivalves of Mid- Jurassic age. Brown sandstones of Mid-Jurassic age were observed by Norin (1946, 1976) on the Loqzung Mountains.

A limited strip of 'coarse littoral clastic rocks, intercalated with terrestrial sediments' occurs in West Qiangtang (Shi & Yang 1986).

Matte et al. (1990, 1992) reported basalts and thin-bedded marine Jurassic limestones on- lapping the Permian sediments south of Lake Longmu. Marine conditions recovered during Aalenian-Early Bajocian times.

Southeast Pamir and Wakhan. Conglomerates and sandstones characterize the succession of the so called 'Axial Zone' of Southeast Pamir in the latest Triassic (Dronov et al. 1982). Coal and sandstones occur in the Toarcian-Aalenian, whilst the whole Early Jurassic is commonly missing (Dronov et al. 1959). Marine trans- gression took place in the Bajocian or later in the Callovian (Dronov et al. 1959).

In some areas external to the 'Axial Zone', like the Hyrymdinsk Zone, sandstones are con- fined to the Hettangian (Dronov in Dronov & Leven 1976). According to Leven (pers. comm., 1992) in the Southeast Pamir 'Intermediate Zone' of Dronov et al. (1982, p. 110, Fig. 5) Sinemurian sandstones overlie with angular un- conformity a flyschoidal succession of latest Triassic age. Sandstones are in turn covered by Toarcian-Aalenian shallow-marine carbonates.

In the Alichur mountains, Middle and Upper Triassic basalts are covered by a melange with Triassic calcareous olistholiths, in turn overlain with angular unconformity by Early Jurassic marine sediments (Dronov & Leven 1990).

All of these data from Southeast Pamir point to deformation and uplift, shifting through time from the Late Triassic to the Early Jurassic.

In Wakhan, Kafarski & Abdullah (1976) describe a succession of thin terrigenous de- posits, passing upward to oncolitic limestones of Early-Mid-Jurassic age, which rests uncon- formably on Lower Triassic pelites.

Rushan-Pshart area and Central Pamir. The Jurassic succession up to the Bajocian consists of sandstones and conglomerates; a shallow- marine ingression is recorded in the Bathonian (Dronov in Dronov & Leven 1976). In the Rushan-Pshart area, Lower Jurassic siltstones contain plant fragments (E. Leven, pers. comm. 1992).

Central Afghanistan. The complex evolution of Central Afghanistan has been recently reviewed by Boulin (1981, 1988, 1990). Three main tec- tono-stratigraphic elements may be dis- tinguished south of the Herat Fault, which may be considered as the Palaeotethys suture (Sen- g6r 1984): (1) the Turkman and Band-e-Bayan zones to the north, where the Permo-Triassic succession is folded and unconformably overlain by a terrigenous apron of Early to Mid-Jurassic age, fed by the so called 'Hercynian Belt' emerged in the north; (2) the Farah Rud zone, interpreted as a complex back-arc area floored by oceanic crust (Waser-Panjao Ocean); (3) the Helmand block, which during the Early Jurassic recorded rifting and platform drowning, but not compressional movements.

Discuss ion

Everywhere in neighbouring areas there is evi- dence of an erosional event linked to com- pression and uplift, which may be ascribed to the 'Eo-Cimmerian' orogeny (Seng6r 1984). Open questions are of two types.

Timing. Terrigenous deposition began either in the latest Triassic or in the late Early Jurassic to early Mid-Jurassic. Although there is need for greater precision of age information, two dis- tinct events are recognized. Sandstone petro- graphic data are unfortunately largely lacking from these adjoining areas.

The North Karakoram succession recorded both episodes. Provenance of detritus from an accretionary prism containing oceanic rocks suggests an arc-continent collision scenario for the older Early Jurassic episode. In Southeast Pamir, uplift and erosion started in the latest Triassic, i.e. somewhat earlier. The younger Pliensbachian-Bajocian episode in the North Karakoram is documented by 'molassic' de- position sealing the deformed continental mar- gin between the latest Pliensbachian and the Late Aalenian.

Provenance. The overall picture we gain from Afghanistan and Pamir is that subduction and uplift occurred in the north (present position) with terrigenous detritus carried to the south, onto the southern passive margin represented by the Helmand Block. In Chapursan and partly in East Karakoram and Shaksgam, the available data suggest instead that deformation began and was more active in the south. Can this be due to late-stage strike-slip faults which inverted the original palaeogeography? Or do we have to

50 M. GAETANI ET AL.

start searching for another 'Eo-Cimmerian' oro- genic belt in the south?

The Permian evolution of the Chapursan area strongly resembles that of the Helmand Block in Central Afghanistan, even though the latter received terrigenous detritus only at a late stage and from the north, by-passing the Farah Rud Zone (Boulin 1988). The problem of where the accretionary prism was located remains open, with the further possibility of long-distance longitudinal transport of detritus.

The mid-Cretaceous orogenic event

The Chapursan sedimentary succession was strongly faulted and folded at very low meta- morphic grade prior to deposition of the fan delta Tupop Conglomerate, which sealed an highly irregular topography. Noteworthy is the lack of Orbitolina limestones and granitoid clasts.

Marine sedimentation continued until the Late Cretaceous in a very active geodynamic setting.

Compar i sons with nearby areas

North Karakoram. Two belts of granitoid bodies with late Early Cretaceous radiometric ages occur south and north of the Chapursan sedi- ments. The northern belt (Khunjerab, Giraf, Shuijerab, Sughet Jangle granitoids, plus the K2 Orthogneiss) has been the subject of only pre- liminary studies. Available ages (K/Ar, Ar/Ar on biotite or hornblende, U/Pb on zircon) cluster between 120 and 105 MA for crystalliz- ation, and up to around 70MA for cooling (Treloar et al. 1989; Searle et al. 1990; Le Fort in Gaetani et al. 1991; Crawford & Searle 1992; Debon 1992). In the southern belt, representing the earliest known magmatic pulses of the Axial Batholith (e.g. Hunza pluton), U/Pb zircon ages of 95 +_ 5 MA were obtained (Le Fort etal. 1983; Debon et al. 1987).

Hindu Raj and East Hindu Kush. The presence of Orbitolina limestones and derived pebbles in the Reshun Conglomerate has been known since Hayden (1915). Additional occurrence of Aptian-Albian rudist remains was reported by Talent et al. (1981) and Pudsey et al. (1985). In the Karambar valley, Casnedi (1980) reported Orbitolina pebbles in the Reshun Conglomer- ate, but no Orbitolina limestones.

Shaksgam valley and East Karakoram. The Urdok Conglomerate has been tentatively re-

considered as Cretaceous by Gaetani et al. (1990b, 1991). The earlier Desio (1980) assign- ment to the Middle Triassic seems untenable, since deep-water carbonates were deposited in the area at those times. We consider the Urdok as a possible equivalent of the Qizil Lunghar Conglomerate, cropping out at the southern border of the Depsang Plateau. Dainelli (1933, Chisil Lungur in his spelling) suggested a Norian age, since the conglomerate rests on fossiliferous Carnian deposits. Cretaceous microfossils, how- ever, were found in the matrix (Gergan & Pant 1983; Gergan 1989 pers. comm.).

West Tibet. Strong deformation affected the sedimentary succession prior to marine trans- gression in the Cenomanian (Parona 1928; Dainelli 1933). Shallow-marine rudist-bearing limestones were deposited at least until the Turonian (Parona 1928). In other areas, con- tinental red beds directly cover the folded suc- cession.

Southeast Pamir and Wakhan. Lowermost Cre- taceous marine sediments are unknown in this area (Shvolman 1973), where marine conditions probably ended in the Oxfordian (Dronov in Dronov & Leven 1976). The sequence mostly consists of tufts, volcanoclastics, sandstones and conglomerates deposited in subaerial environ- ments.

In the Tas Kupruk Zone of Wakhan, Kafarski & Abdullah (1976) describe red conglomerates (200 m) followed by some 500 m of purplish siltstones, tentatively attributed to the Early Cretaceous. They rest with marked angular unconformity onto Palaeozoic rocks.

Central Afghanistan. The Central Afghanistan blocks were uplifted in the Barremian-Aptian with emergent areas and Orbitolina-rich shallow seas occurring both in the Band-e-Bayan and Helmand blocks after closing of the Waser- Panjao Ocean in the course of the Neocomian (Boulin 1990). Tectonically-activated trenches started to be filled with terrigenous sediments in the Cenomanian. In the Senonian, shallow- water limestones prograded onto the peneplaned 'Neo-Cimmerian' range, both from north and south (Boulin 1990).

Discuss ion

In the Chapursan area there is undisputable evidence of the same mid-Cretaceous event which strongly affected all neighbouring blocks ('Neo-Cimmerian orogeny'; Seng6r 1984). The lack of Orbitolina limestones might be ascribed

MESOZOIC HISTORY OF THE NORTH KARAKORAM 51

to a still too deep environment during the Barremian. The finding of small Protopeneroplis in unit 6, however, may suggest that shallow seas existed in the earliest Cretaceous, but they did not last until the Barremian. The area thus possibly emerged at an early stage, suggesting an intermediate position between Southeast Pamir and East Hindu Kush.

Notable is the apparent lack of major related volcanism, which is instead widespread in Pamir and Helmand, even though the conglomerates directly overlie basalts of unknown age and do contain common felsitic volcanic rock frag- ments.

At the present state of knowledge we are still uncertain whether deformation was caused by northward subduction of Neotethys in the south, leading to collision with the Kohistan-Dras arc and formation of the Kohistan Northern Suture during the mid-Cretaceous (Petterson & Wind- ley 1985; Coward et al. 1986; Searle 1991), or to subduction underneath a northern arc, feeding the Khunjerab Granitoid belt (as suggested by Debon et al. 1987).

Peculiar to the Chapursan area is the de- position of relatively deep marine sediments after the main deformation event. They docu- ment still active faulting, possibly related to the first stages of back-arc compression and strike- slip motions that characterized the structural evolution of the Karakoram Range during the Tertiary.

Paper supported by the project 'Geology of Karako- rum', leader M. Gaetani. EEC Project CI*CT 90/ 0852. L. Angiolini and A. Zanchi assisted in the 1991 field work. M. Gallo organized logistics with great capability. I. Premoli Silva kindly identified some of the benthic foraminifera. Field-work in 1991 was carried out by M. Gaetani and F. Jadoul. E. Erba studied the nannofossils and E. Garzanti, who also did field-work in the 1986 expedition, the sandstone petrography. The manuscript has benefitted from careful comments by C. Pudsey and A. Zanchi. Special permission given by the Government of Pakis- tan to visit restricted areas is acknowledged.

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