Chap 4

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(Scotese and Barrett, 1990). The warm Cambrian period following the. Vendian glaciation witnessed the greatest explosion of life. Aided by sea-level rise and ...


Early Palaeozoic palaeogeography, basin configuration, palaeoclimate and tectonics in the Indian Plate O.N. BHARGAVA 103, Sector 7, Panchkula 134109, India Email: [email protected]

Abstract Cambrian sequences are exposed in Bikaner (peninsular India), the Salt Range, the Lesser Himalaya, Hazara (Pakistan), Himachal, Garhwal and possibly Nepal and Arunachal Pradesh. They are found also in the Tethyan Himalaya in Kashmir, Spiti-Zanskar, Kinnaur-Kumaon and Bhutan; due to metamorphism and plutonism, the Cambrian of Nepal has not been clearly discriminated. The Cambrian in Kashmir ranges up into the early Late Cambrian and in Bhutan to middle Late Cambrian; in other areas the youngest beds are Middle Cambrian. The Cambrian sequences accumulated primarily in subtidal-supratidal environments. A submarine ridge is hypothesized to have existed between during earliest Cambrian between the Lesser Himalayan and Tethyan basins. This ridge hosted Early Palaeozoic granites developed in connection with Cambrian tectonism; de-roofing of them resulted in Late Precambrian-Early Cambrian zircons being shed into both basins. The Cambrian Event produced folding of Cambrian and older sequences and thrusting of Proterozoic sequences over the Cambrian Chilar Formation¯exposed in the windows in the Tons Valley. Extensive regression during the late Cambrian culminated in total withdrawal of the sea. Apart from Bhutan, the Early Ordovician marine transgression over the folded Cambrian terrain was restricted to the Tethyan region. Sedimentation, commencing with conglomerate followed by coarser arenites in all the areas, accords with sharp rise of the subaerial ridge in the southern part of the Tethyan basin. By the Late Ordovician, the Bhutan region too was inundated and almost in all the basins, with the possible exception of the Peshawar region, small coral-algal-stromatoporoid buildups of Ashgill-Wenlock age were formed. Sedimentation in most of the area ceased during the LlandoveryWenlock, but in the Peshawar region it persisted into at least Emsian times. Llandovery graptolite faunas indicate that part of the Nepal area included a distal basin. The Cambrian tectonism, during the Middle Cambrian times, initially raised the submarine ridge into a subaerial ridge. This was followed by thrusting in the Tons Valley, sharp uplift of the ridge in the Late Cambrian, and raising of the Early Ordovician basin. Its last impulse seems to have been



reflected in soft sediment deformation (palaeoseismites) affecting the late Ashgill-Llandovery in the Parahio and Pin valleys of Spiti and Uttarkhand. Keywords: Cambrian basin, submarine and subaerial ridge, tectonics, Cambrian-Ordovician angular unconformity, Early Ordovician conglomerate, Ashgill-Llandovery/Wenlock buildups, Bikaner, Salt Range, Lesser Himalaya, Tethyan Himalaya.

INTRODUCTION During the Vendian and earliest Cambrian the continents amalgamated to generate a single supercontinent Rodinia (name derived from the Russian ‘rodina’ meaning ‘homeland’). At the beginning of the Cambrian, Rodinia fragmented into smaller continents. Melting of the Vendian ice masses resulted in sea-level rise. Most of the continents during this time lay in tropical, subtropical, and temperate climatic zones with, during the Cambrian and Early Ordovician, the South Pole located adjacent to northwest Africa (Scotese and Barrett, 1990). The warm Cambrian period following the Vendian glaciation witnessed the greatest explosion of life. Aided by sea-level rise and abundant food supply connected with pronounced volcanic activity and ecologic changes, biodiversity increased dramatically in what has been referred to as the ‘great Ordovician biodiversification event’ (Webby et al. 2004). Continental dispersal reached a maximum for the Palaeozoic during the Late Ordovician; it was presumably connected with swift sea-floor spreading. Warm (greenhouse) climate extended from the Cambrian into the Ordovician; the pole migrated into the Amazon basin during the late Ordovician. The greenhouse period was interrupted by the Hirnantian glaciation near the end of the Ordovician (Brenchley et al. 1994); it led to major mass extinctions (Sheehan, 2001). Atmospheric CO2 levels during the Cambrian-Ordovician were 15 times more than the present day (Berner and Kothavala, 2001). The last part of this was a period of reefal activity. During the Silurian period, there were two main landmasses, with the South Pole located in Argentina (Scotese and Barrett, 1990). The southern continent (Gondwanaland) was chiefly located south of the equator; it consisted of South America, Africa, Madagascar, India, Australia and Antarctica. The northern macrocontinent (Laurasia) consisted of North America, Greenland, Europe and most of what presently consists of Asia. The end of the Hirnantian glaciation heralded warm climate. Glaciers, like those of the present day, became confined to higher latitudes. Following the Hirnantian (Late Ordovician) mass extinction, invertebrates recovered



rapidly; diversity increased swiftly. Coral-stromatoporoid-bryozoan-algal reefs flourished in warm seas. The first land plants, as indicated by the record of spores, which had appeared in the Llanvirnian, diversified through the Silurian. The Silurian also witnessed the invasion of land by animals. Cambrian, Ordovician and the Silurian sequences have limited development on the Indian Plate. Though Cambrian sequences, outside the Himalaya, occur in the Salt Range and Rajasthan, Ordovician and Silurian sequences are mainly restricted to the Tethyan Himalaya (Fig. 1). The Cambrian on the Indian Plate consists mainly of siliciclastic sequences with minor carbonates. The Early Ordovician is

Fig. 1. Location of Early Palaeozoic seqeunces in Bikaner and Salt Range of Cambrian is present

represented by coarser clastics, whereas the Late Ordovician was dominated by carbonates and buildups. The OrdovicianSilurian boundary in the Himalaya is nowhere tightly constrained by palaeontologic data. Silurian sedimentation extended into the Llandovery and perhaps the Wenlock, though in the Peshawar region sedimentation extended without interruption into the Emsian and perhaps appreciably later. Only generalized thickness and palaeocurrent directions are available for the Early Palaeozoic, and fossils with precise bathymetric constraints are rare. Thus, delineation of palaeoshorelines is at best conjectural. A brief account of Early Palaeozoic successions, useful for reconstructing basinal configurations follows. CAMBRIAN Sustained investigation by Nigel Hughes and associates (Hughes, 1997; Jell and Hughes, 1997; Hughes et al. 2005, 2010; Peng et al. 2009) have



resulted in well-defined biostratigraphies of the Cambrian of the Himalaya. Cambrian sequences are preserved in the Tethyan domain in the Kashmir, Spiti-Zanskar, Kinnaur-Kumaon, Nepal and Bhutan synclinoria as well as in the Peshawar region. They are also preserved in Rajasthan, the Salt Range, the Lesser Himalaya of Hazara (Pakistan), Himachal and Uttarkhand and possibly in Nepal and Arunachal. (Fig. 1).

Peninsular part Strictly speaking the Salt Range belongs neither to the Himalaya nor to the peninsula region. In the present communication, however, it is classified under the peninsular part due to its proximity to and similarity with the Bikaner Basin; both are underlain by evaporite facies. i) Bikaner (Rajasthan): The Nagaur Formation forming part of the Marwar Supergroup (= Vindhyan Supergroup) is divisible in the Nagaur Sandstone and the Tunklian Sandstone. The former consists of brick-red sandstone, siltstone, shale and greenish clay bands; it shows cross-bedding, parallel bedding, ripple marks and shrinkage cracks. Well-preserved trilobite and the trace fossils of the Cruziana Facies, including Dimorphichnus, and Rusophycus as well as Aulichnites, have been obtained from the Nagaur Sandstone (Kumar and Pandey, 2008). These are all long-ranging forms, but the absence of body fossils is consistent with an Early Cambrian age for the assemblage. The overlying Tunklian Sandstone consists of gritty and pebbly sandstone. Though the Cruziana Facies has been considered to indicate the sublittoral zone, the presence of cross-bedding and ripple marks with shrinkage cracks in sandstones indicates sandy tidal-flat environments with occasional subaerial exposure. The overlying gritty to pebbly sandstone of the Tunklian Sandstone indicates further shallowing in a nearshore setting. It seems that the Bikaner shoreline could not have been far from the presentday outcrops of the Nagaur Formation. ii) Salt Range: The Cambrian succession has four distinct sequences: (i) about 80 m of thick reddish-brown to purple, thick-bedded to massive ripple marked and mud-cracked sandstone with minor brown shale intercalations (Khewra Sandstone = Purple Sandstone). It is devoid of body fossils but contains trilobite trace fossils consistent with being earliest Cambrian (Schindewolf and Seilacher, 1955); (ii) a thin but widespread conglomerate bed followed by grey silty and sandy glauconitic shale, minor sandstone and a few black shale horizons (Kussak Formation = Neobolus Shales).



Thicknesses vary between 30 m and more than 200 m; it has produced Neobolus warthi and Redlichia noetilingi indicating an Early Cambrian age; there is possibly a minor hiatus between the Khewra Sandstone and the Kussak Formation; (iii) a sequence of dolomite with minor shale intercalations (Jutana Formation) with Redlichia noetlingi and Pseudotheca subrugosa of Early Cambrian age; (iv) reddish-brown shale, ripple-marked and mudcracked flaggy sandstone with abundant halite casts (Baghanwala Formation = Salt Pseudomorph Beds) of possible Middle Cambrian age. The entire sequence seems to have been deposited in a gradually shallowing inter-tidal to tidal basin leading to desiccation and formation of halite casts.

Lesser Himalaya The Cambrian sequences are present in Hazara, Pakistan (Latif, 1970, 1974), Himachal (Nigalidhar and Korgai synclines), Uttarkhand (Mussoorie and Garhwal synclines), Bhutan and also possibly in Nepal. i) Hazara (Abbotabad, Pakistan): The Sirban Formation carbonate sequence (~200 m) with phosphorite bands with Hyolithes spp. and Hyolithellus spp. has been referred to the Early Cambrian (Shah, 1977). The overlying sequence, (c. 300 m) consists of purple quartzitic siltstone (with minor manganese oxide layers) overlain by haematitic shale, sandstone, and quartz breccias with local volcanic rocks (Hazira Formation). The lower part is glauconitic and the middle, from which small shelly fauna has been reported, is phosphatic (Fuchs and Mostler, 1972). Both the Sirban and the Hazara basins appear to have been shallow and barred with frequent phosphate-rich upwelling currents from the open sea (cf. discussion below under the Tal Group). ii) Himachal and Uttarkhand: The Cambrian of these areas has been referred to as the Tal Group; it is exposed in the Nigalidhar, Korgai (Himachal), Mussoorie and Garhwal (Uttarkhand) synclines (Fig. 2). Three formations have been discriminated. When fully developed, the lowest or Shaliyan Formation commences with siltstone, shale and limestone weathering to earthy colour; volcaniclastic input as well as pyrites occur locally. This part is overlain by chert and phosphorite with, locally, limestone. A small shelly fauna (SSF) has been reported from the cherts and phosphorites (Brasier and Singh, 1987). The Anabarites trisulcatus-Protohertizina anabarica assemblage (Bhatt et al., 1985 and references therein) of the Meishucunian Stage (Early Cambrian) is the most important and widely distributed SSF.



Fig. 2. Simplified geological map of parts of Himachal and Garhwal Himalaya. Jaunsar-Krol and Cambrian (Lesser Himalaya) constitute Inner Krol Belt.

The succeeding Sankholi Formation is a siltstone-shale sequence with local load and flute casts; it is succeeded by parallel to low-angled cross-bedded siltstone with carbonate bands in its upper part. From carbonate bands, Kumar et al. (1987) reported Dimidia and Allonnia. The presence of Dolerolenus (Malungia) cf. M. laevigata, Drepanopyge gopeni, Protolenella cf. P. angustilimbata (oldest trilobites so far known from the Himalaya) and Redlichia noetlingi from the upper part of the Sankholi Formation restricts its age to Tsanglangpuian (Hughes et al., 2005). The upper part (Koti Dhaman = Deo ka Tibba Formation) consists of white to grey sandstone followed by reddish grey shale, pale to white felspathic sandstone, grey stromatolitic limestone and well sorted to fairly well sorted white sandstone (Bhargava, 1976) From this part, Redlichia noetlingi and Xela mathurjoshi have been reported (Kumar et al., 1987; Mathur and Joshi, 1989; Hughes et al., 2005). The upward sequence from chert and shale to phosphate is considered by Shanker (1987) to represent regressive and transgressive phases of the sea. His surmise that the presence of pyrite, radioactive elements and organic matter indicate a reducing environment with negative Eh has been corroborated by detailed work by Mazumdar et al. (1998). Based on marked lowering of δ13Ccarb and δ13Corg and a Ce anomaly in the Tal Phosphorite, Mazumdar et al. (1998)



suggested upwelling currents from the deeper open sea during deposition of the phosphate. The remaining Tal succession represents sedimentation on a stable shelf of a shallow tidal sea mainly in a lagoonal, sandbar-shoal complex and shelf mud (Singh et al. 1980 in Shanker, 1987). Abundance of stromatolites may indicate extremely shallow marine conditions. In addition to the Tal Group, the Cambrian is represented by micaceous arenites with abundant Skolithos of the Chilar Formation (Rai et al. 1997). It is exposed as windows and tectonic slivers within the Palaeoproterozoic Dharagad Group in the Tons Valley (Fig. 3), NE of the Nigalihar-Korgai synclines and NW of the Mussoorie Syncline. The micaceous nature of the arenites indicates lack of winnowing, whereas the profuse development of Skolithos accords with deposition in a sandy shore environment. iii) Nepal: Fuchs (1967) reported Krol and Tal in Nepal, but no Cambrian fossils have been found in that region. Sakai (1983) reported trace fossils from the Saidi Khola Beds (Angha Khola Formation), a 200 m sequence of quartzose arenites and shale, the former dominating the basal part of the unit. No body fossils are known from this sequence. Bioturbation and trails are restricted to the upper shaly part of the Saidi Khola Beds. This sequence may correlate with the Chilar Formation of the Tons Valley. Not enough data are available to reconstruct the depositional environment of the Saidi Khola Beds. iv) Arunachal: Planolites and Skolithos have been reported from the Miri Formation (Tandon et al. 1979). Absence of body fossils in this arenaceous sequence suggests that it may equate with the Chilar Formation of Himachal and the Saidi Khola Beds of Nepal.

Tethyan Himalaya In view of their similar lithologic assemblages and fossil contents, these occurrences in various synclinoria are considered to have been parts of a once continuous basin, though submarine basinal highs between various synclinoria could have existed. i) Peshawar and Attock-Cherat Ranges: Dolomite with chert lenses and minor shale (Amber Formation) and limestone with maroon argillite (Darwaza Formation) have been assigned a Cambrian age. A Cambrian age is suggested because both sequences though unfossiliferous are overlain by the Ordovician conglomerate (Kazmi and Kasim Jan, 1997).



Fig. 3. Map of a part of the Tons Valley showing locations of the Chilar Formation as tectonic slivers and as windows within the Dharagad Group.



ii) Kashmir: The Cambrian sequences of Kashmir developed in Hundwara (Sind Valley) and the Lidder Valley (Middlemiss, 1910; Shah, 1968) have been designated the Hapatnar Group (Bhargava, 2008a). The lowest (Lolab) consists of siltstone, shale and fine-grained sandstone; its upper part contains Redlichia takooensis (Tsanglangpuian) in the basal part and Chittidilla plana and Yuehsienszella szechuanensis (Lungwangmiaoan) in its upper part, both indicating an early Cambrian age. The overlying sequence (Shumal Formation) consists of olive green-gray shale siltstone. Xingrenaspis dardapurensis, Baltagnostus cf. rakuroensis, Pagetia sp., Tonkinella breviceps, Bailiella lantenoisi, Parachittidilla kashmirensis, Shahaspis himalayensis, Latilorenzella sp., Hundwarella memor, Iranolessia butes (Maochuangian-Hsuchuangian, Middle Cambrian) are known from this formation (Jell and Hughes 1997). The uppermost sequence, the Rangmal Formation, consists of shale, siltstone, quartzarenite and limestone with bivalve, gastropod and algal remains. From the Rangmal Formation, Damesella shergoldi (Changhian) in the basal part, and Cyclolorenzella sp, Monkapsis cf M. serrata and Blackwelderia sp (Kushanian- early Late Cambrian) from upper part, have been reported (Jell and Hughes, 1977). Parcha (1998) reported an Early Cambrian ichnofauna that includes Trichophycus pedum from the Lolab and Rangmal formations. The latter formation is possibly an expanded version of the Parahio Member of the Kunzam La Formation of Spiti and Zanskar. The Cambrian sequence in Kashmir ranges in age from earliest Cambrian to early Late Cambrian (Jell and Hughes, 1997). Sedimentation in Kashmir part took place, in part, in an environment ranging from subtidal shoreface to tidal flats. iii) Spiti-Zanskar: The Cambrian is represented by the Haimanta Group, divisible into the Batal and Kunzam La formations. The PrecambrianCambrian boundary is located in the upper part of the Batal Formation (Bhargava et al. 1982). The basal part of the Kunzam La Formation consists of shale and siltstone (Debsa Member) overlain by shale, siltstone, arenites and beds of limestone (Parahio Member). The thickness of the limestone beds increases west of Spiti into Lahaul and Zanskar where it is mappable and has been designated the Karsha Formation. The sequence above this, termed the Kurgiakh Formation, is shale-rich in the basal part with black-grey shale, silt-streaked shale and minor orange dolosiltite and dolomicrite with irregular chert stringers (Surichun Member); it is overlain by interbedded sandstone and silt-streaked shale (Kuru Member) (Myrow et al. 2006a). The thickness of the Cambrian succession in Zanskar is around 600 m (Myrow et al. 2006b); in Spiti at Kunzam Pass it is around 1200 m, and in the Parahio



Valley, according to Bhargava and Bassi (1998), 1100 m; Myrow et al. (2006b) estimated its thickness in the Parahio Valley as around 1300 m. The trilobite fauna in Spiti-Zanskar is represented by Redlichia noetlingi (Tsanglangpuian), Pagetia significans, Oryctocephalus indicus, Kumingaspis pervulgata (Lungwangmiaoan), K. stracheyi, Shantungaspis himalaica, Paramecephalus defossus (Maochuangian), Opsidiscus haimantensis, Oryctocephalus salteri, Xingrenaspis maopoensis, Solenoparia talingensis, Douposiella hostilis, Hundwarella memor, Iranolesia butes, Xingrenaspis dardapurensis (Maochuangian Hsuchuangian, Middle Cambrian), Torifera sp, Damesops sheridanorum, Schmalenseeia amphionura, Fuchouia sp, Clavagnostus repandus, Linguagnostus cf. tricupis, Hypagnostus correctus, Lejopyge armata, Goniagnostus aculeatus and Szeaspis conjuntiva (Changhian). The trilobites indicate an age range from Tsanglangpuian to Changhian (Jell and Hughes, 1997; Parcha, 1998). In addition to these, Peng et al. (2009) have described a diverse collection of trilobites; the more important among them are Fuchouia oratolimba, Haydenaspis parvatya, Kaotaia sp. cf. K. gedongensis, Mufushania nankingensis (Stage 4 of Cambrian System), Opsidiscus haimantensis, Bhargavia prakritika, Douposiela himalaica, Eosoptychoparia sp. Gunnia smithi, Kaotaia prachina, Kunmingaspis stracheyi, Monanocephalus maopoensis, Paramecephalus defossus, Probowmania bhatti, Xingrenaspis hoboi, X. shtamalae, X. parthiva, Ziboaspis hostilis, Solenoparia taligensis, Proasaphiscus simony, Hundwarella memor, ?Iranolesia butes, Sudanomocarina sinindica, Peronopsis acadica, (Stage 5 of the Cambrian System), Ammagnostus cf. A. laiwuensis, Proagnostus bulbus bulbus, Clavagnostus calensis, C. trespinus, Utagnostus neglectus, Diplagnostus planicaudia, Baltagnostus rakuroensis, Hypagnostus brevifrons, Ptychagnostus aculeatus, Goniagnostus sp, Lejopyge acanthi, L. armata, Tomagnostella exsculpta, Valenagnostus imitans, Parablackwelderia jimaensis, P. sheridanorium, P. yangi, Koldinia odelli, Torifera jelli, Himalisania sudani, Neoanomocarella asiatica (Guzhangian Stage). Srikantia (1981) considered the Cambrian sediments (Kunzam La Formation) both in Spiti and Zanskar to be flysch. Subsequently, Garzanti et al. (1986) regarded the upper part of the Cambrian (Kurgiakh Formation) as flysch. Bhargava and Bassi (1998) interpreted the Cambrian of the Spiti Valley to represent mainly subtidal to intertidal depositional environments. Myrow et al. (2006a, b) advocated a siliciclastic deltaic setting containing numerous medium-scale shoaling cycles ranging from storminfluenced offshore deposits to thick trough cross-bedded fluvial facies where



carbonate beds with rich trilobite faunas represent transgressive-system-tract deposits. iv) Kinnaur-Uttarkhand: In Kinnaur, the term Kunzam La Formation has been used for the Cambrian sequence whereas in the Uttarkhand part it is referred to as the Milam Formation (Kacker and Srivastava, 1996). In Kinnaur, the lithology of the Kunzam La Formation is comparable with the Debsa Member; equivalents of the Parahio Member have possibly been eroded; the Milam Formation is more arenaceous and includes lenses of carbonates in Uttarkhand. Redlichia noetlingi has been reported from the Milam Formation (Kacker and Srivastava, 1996; Srivastava et al. 1996) whereas only trace fossils are known from Kinnaur (Bhargava and Bassi, 1998). iv) Nepal: Little information is available on the Cambro-Ordovician rocks of the Tethyan zone of Nepal; they are metamorphosed and have produced a very few taxonomically inconsequential brachiopods (Fuchs, 1967, 1977). Similarly insignificant fossil fragments have been reported in the metamorphics of the Jomsom and Manang areas. There is a general consensus that the Dhaulagiri Limestone (Dolpo area; Fuchs, 1967, 1977), the Nilgiri Limestone (Jomsom area; Bodenhausen et al. 1964; Bordet et al. 1971) and the Yellow Formation of Annapurna in Manang (Bordet et al. 1976) are at least partly Cambro-Ordovician. In Kathmandu, an outlier of Ordovician-Silurian rocks containing trilobites, brachiopods and other fossils (Bordet et al. 1967; Stöcklin, 1980) occurs; Cambrian rocks have not been identified; all reports of Devonian rocks from the area have been demonstrated to be spurious (Talent et al., 1988, 1991; Talent, 1989; Webster et al., 1993). Since the Cambrian sediments are exposed in Uttarkhand to the west and Bhutan to the east, their presence in Nepal seems highly likely. It seems much of the Cambrian succession has been lost to Early Palaeozoic granites and metamorphism (written comm. Dr M. Dhital, Tribhuvan University). v) Bhutan: The Cambrian comprises quartz-rich sandstone with shale drapes showing trough cross-bedding, desiccation cracks, small ripples and abundant bedding-parallel burrows (Deshichiling Formation), shales with thin finegrained sandstone (Maneting Formation), hummocky cross-stratified sandstone with parallel lamination, thin-bedded grey fine grained sandstone with profusely bioturbated grey-green shale and limestone showing hummocky cross stratification (Quartzite Formation) (Hughes et al. in press). Earlier, the



Deshichiling Formation was assigned a probable Late Precambrian age (Tangri and Pande, 1995). The sequence of the Quartzite Formation represents high-energy shallow marine environments (with periodic exposure) representing a prograding storm-dominated shoreline deposit (Hughes et al. in press). The fossils in the Maneting Formation are: Kaolishania granulosa, Taipaikia glabra, Lingyauanaspis sangae (trilobites) and Billingsella cf. tonkiniana (brachiopod). The fauna is correlated with the Kaolishania Zone, Stage 9, of the Cambrian System, and Furongian Epoch of the North China Block (mid-Late Cambrian); it represents the youngest Cambrian horizon in the Himalaya (Hughes et al. in press). Hughes et al (in press) consider the Bhutan Cambrian to differ from that of the Spiti-Zanskar region due to the presence of Stage 9 of the Cambrian System. Lithologic assemblages of the underlying Maneting and Deshichiling formations closely resemble those of the Kunzam La Formation. A thorough search may lead to discovery of Early and Middle Cambrian trilobites making them broadly equivalent to the Debsa and Parahio Members respectively of the Kunzam La Formation. It would thus be appropriate to say that a younger horizon than any in the Kashmir and Spiti-Zanskar region is preserved in Bhutan. There could be two interpretations regarding the presence of younger horizons in Kashmir in the extreme west and Bhutan in the extreme east: i) there was uneven erosion and it was maximal in the central Spiti-ZanskarKumaon-Nepal region and less in the two extreme ends of Kashmir in the north-west and Bhutan in the east; ii) the Cambrian regression was gradual, commencing in the Early Cambrian of Bikaner, in the Middle Cambrian of the Salt Range and the Spiti-Zanskar-Kumaon-Nepal region and, in the Late Cambrian in the farthest corners of Kashmir and Bhutan. It might also be a combination of both processes, i.e. gradual regression and differential erosion in the central regions reflecting the earlier marine regression (Bhargava, 2008a).

Basin configuration, tectonics and palaeoclimate It is possible that Cambrian fluvial and/or estuarine sediments might have been deposited in the main Vindhayan basin (cis-Aravalli) and have since been eroded. The available stratigraphic record, however, cannot confirm this. The Bikaner region is hypothesised as probably the extremely shallow easternmost extension of the Cambrian sea. From Bikaner, the sea is interpreted as having stretched westwards towards the Salt Range and,



farther west, to have merged with open seas. To the north, the sea is viewed as probably having extended in the Lesser Himalaya. Before the shape of the basin is interpreted, the following aspects need consideration: i) Kashmir and Spiti-Zanskar synclinoria: The Kashmir Synclinorium lies southwest of the Spiti-Zanskar Synclinorium towards the craton (i.e. up depositional dip). After correction for crustal shortening, the Kashmir region would be ~500 km up-depositional dip. Because of this suggested situation, the Kashmir region should have displayed shallower lithofacies compared with the Spiti-Zanskar region. However, upslope to the pre-Panjal level, the lithofacies of both the basins are highly comparable. It is only from the Permian onwards that the Kashmir region displays shallower environments compared with its counterparts in Spiti-Zanskar. Situated closer to the craton, the Cambrian regression should have affected the Kashmir region earlier and, if the presence of younger horizon is explained by differential erosion, the Kashmir region, relatively proximal to the craton should, in normal circumstances, have undergone greater erosion. The Cambrian sequence in Kashmir, however, is younger than the Spiti-Zanskar sequence. It was only after the Late Callovian break that either there was no sedimentation in Kashmir or this sequence has been eroded. Based on similar facies in both areas, it was suggested that originally the Kashmir region was a strikeextension of the Zanskar region (Fig. 4) and, during the Permian Gondwana rifting, manifested by the Panjal Volcanics, it was sheared and brought to its present position (Bhargava, 2008a). This shear (the Zanskar Shear) was possibly reactivated during the Himalayan Orogeny. ii) Chilar and Tal basins: The Chilar Formation, correlating with the tracefossil-bearing part of the Tal Group (Rai et al. 1997), is exposed in several windows in the Dharagad Thrust Sheet (Bhargava, 2004; Bhargava et al., 2011) in the Tons Valley, almost north-east of the Tal Group of the Nigalidhar-Korgai synclines. The Tal sequence forms part of the Inner Krol Belt Nappe (sensu Bhargava, 1976); tectonically below it is the Outer Krol Belt Nappe (Fig. 2). The relative position of these, prior to NE-SW thrusting, would have been Tal, Krol, and Chilar (Fig. 5). Detrital micas in the Tal sequence are 860 Ma and those of the Chilar 1840 Ma (Frank et al. 2001) indicating that though these basins were largely coeval, there existed a barrier between them precluding mixing of micas. This barrier could have been the Outer Krol Belt that, prior to the thrusting, occupied a position between the Tal and the Chilar basins (Fig. 5). Though at present the Chilar



Fig. 4. Diposition of Cambrian Basin, originally the Kashmir part existed in strike continuation of the Zanskar-Spiti. Submarine ridge existed between the Lesser Hiamalayan and Tethyan basins.

Fig. 5.

Pre-tectonic positions of the Chilar, Outer Krol Belt and Inner Krol Belt basins

Formation is documented only from the Tons Valley, its basin is likely to have been more extensive; this is indicated by the Saidi Khola Beds in Nepal and the Miri Formation in Arunachal. iii) Tal and Kunzam La basins: It was proposed earlier that these two were separate basins, the Tal Basin having an indirect connection with the main sea (Bhargava et al. 1998). Myrow et al. (2003) suggested a continuous basin existed from the Tal region in the Lesser Himalaya to Kunzam La in the Tethyan region. Such an interpretation runs counter to the following: a. The aerial distance across the basin strike (i.e. along the depositional dip) between the southern most limits of the Tal and the Kunzam La outcrops



is about 150 km (Fig. 2). If corrected for crustal shortening and horizontal translation of the thrust sheets, this distance shall far exceed 150 km. Even with a moderate slope of half a degree of the basin floor, the Kunzam La part of the siliciclastic basin should have been deeper at least by ~75 m as compared to the Tal part. Such a situation does not exist. The sequence of the Tal with marine fossils and its corresponding Kunzam La sequence show comparable depths of basin. The correlatable Redlichia and Lingulella bearing horizons in the Tal and KunzamLa formations in the Lesser as well as in Tethyan basins at that time frame, provide best example of identical depths of basin in both the areas. At no time the Kunzam La part was deeper as compared to the Tal part, which should have been the case had these been parts of one continuous basin. Two far-flung areas situated along the depositional dips of the perceived basin can have comparable depths only if they formed two separate basins. b. The Tal Group in the basal part has phosphorite horizon that is absent in the Tethyan part. As mentioned above, marked lowering of δ13Ccarb and δ13Corg and Ce anomaly in the Tal phosphorite indicate presence of upwelling currents in the Tal from deeper open sea during the deposition of the phosphate (Mazumdar et al. 1998). The upwelling current model bringing phosphate in a barred basin envisages a submarine ridge towards the open sea (Kazakov, 1937). Thus a submarine ridge possibly existed between the Tal and the Tethyan basins and also between the Hazira and the open sea (Fig. 6) to provide ideal conditions for upwelling currents to bring phosphate in the Tal and Hazira basins (Fig. 7). Taking into consideration the gentle slope, both the basins must have had a larger expanse as compared to the present outcrop limits.

Fig. 6. Location of visualised submarine ridge between Lesser Himalaya and Tethyan Basin



Fig. 7. Upwelling currents from open sea depositied phosphate in the Lesser Himalayan Cambrian Basin

Felspar grains and pebbles are found in the sandstone and conglomerates respectively of the Tal Group (Auden, 1934; Bhargava, 1976). Granite pebbles in the size range of 10-15 cm constituting around 5% of the total clasts are known from the Ordovician conglomerate of the Zanskar area (Srikantia, et al. 1977). These observations, particularly the presence of granite pebbles, reveal that granites were exposed close to these two basins. The presence of 525 Ma and 563 Ma zircons in the Tal and the Kunzam La sequences (Myrow et al. 2003) is consistent with the granites from which the felspars in the Tal Group sandstones and conglomerates and the Ordovician conglomerates being of Early Palaeozoic age. Early Palaeozoic granites are extensive in the Vaikrita Group between the Tal and Kunzam La basins. It is thus logical to conclude that in “Phosphorite time” (in response to Cambrian deformation; Bhargava et al. 2011) the submarine ridge was exhumed rapidly, became subaerial (Fig 8), and defining two distinct basins, to the north-east and southwest respectively, one on each side (Fig. 9); the ridge is construed as having contributed zircons and granite clasts to the Tal and the Kunzam La sediments. Across this ridge flowed antecedent rivers that brought material from far off places (Myrow et al. 2010). c. Sedimentologic studies by Myrow et al. (2006a, b) indicate development of fluvial facies in the Cambrian of the Spiti-Zanskar region with northeasterly palaeocurrents (Bhargava et al., 1991) across the strike of the marine basin, presumably (because of the wide expanse of the SpitiZanskar region) connected with numerous rivers. During the corresponding period, the Tal basin, situated farther south towards the craton, should either



Fig. 8. Cartoon to show late Early–Middle Cambrian basin. Posiiton of conjctured subaerial ridge and path of regression of the Cambrian sea.

Fig. 9. Lesser Himalayan and Tethyan basins seperated by a ridge having Early Palaeozoic granites

have become a positive area or have also been a site of fluvial sedimentation. Myrow et al. (2003) in fact suggest the upper part of the Tal Group to have been fluvial. The uppermost Tal is a thick sequence of well-sorted clean quartzite devoid of mud or clay. Not only the uppermost Tal but the entire Koti Dhaman Formation (Bhargava et al. 1998) has identical lithostratigraphy with uniform facies and comparable thickness over a strike length of about 200 km as well as displaying typical marine bedding features (Singh in Shanker 1987; Shanker, 1987) and Skolithos. As a rule, winnowing and sorting is characteristic of beach and tidal flats; fluvial sequences, on



contrary, are not likely to be well sorted and retain identical lithostratigraphy, uniform facies and comparable thickness over a great strike length—in this case 200 km. It is thus difficult to accept that the sediments of the Tal area are fluvial and, moreover, it becomes difficult to fit the fluvial sequence into the regional sedimentologic scenario for the Tethyan region. It thus seems that renewed investigation of the environment of deposition of the Cambrian sediments of these areas is needed for resolving this vexing problem. iv) Bhutan: Hughes et al. (in press) have made a significant observation regarding the similarity of the Bhutan fauna with that of North China; this could be important with regard to global palaeogeographic reconstruction of the Cambrian. The first perceptible indication of a Cambrian deformation, designated the Kurgiakh Orogeny by Srikantia (1977) and Srikantia et al. (1977), was elevation of the submarine ridge into a subaerial one in the late Early Cambrian as indicated by Early Palaeozoic zircons in the Tal as well as Tethyan basins. At that time the ridge seems to have had a steeper slope southwards; evidence for this comes from coarser clastics including conglomerates in the Tal Group. ~2500-3000 m of the Tal sediments indicates that the basin originally extended far beyond their present outcrop limit. Palaeo-current directions in the Tal Group are towards the north; its northern extension, south of the above-mentioned ridge, where currents might have been towards the south, seems to have been eroded. Approximate shapes of the visualised Early and Middle Cambrian basins are depicted in Figs. 6 and 8. Global paleogeography of the Cambrian, presence of halite casts in the Salt Range and development of carbonate with Epiphyton in the Tethyan region accords with a warm climate. Prominence of clastics in the Cambrian may point to a humid climate with accelerated erosion sustaining major rivers to transport huge amount of detritus to the basin. That locally, however, the climate could have been arid is indicated by halite casts in the Salt Range.

ORDOVICIAN There are no known occurrences of latest Cambrian in the entire Himalaya; an angular unconformity separates the Cambrian and Ordovician sequences in the Spiti Valley (Hayden, 1934). Marine transgression after the Cambrian Orogeny was confined to the Tethyan part. Ordovician strata are not known in the peninsular or Lesser Himalayan areas though, based on acritarch remains, an Ordovician sequence has been reported from the



Ganga Valley (Prasad et al. 2001). As neither macrofossils nor bioturbation has been documented from the Ganga Valley, this report needs confirmation.

TETHYAN HIMALAYA i) Peshawar: The Misri Banda Quartzite (175 m) rests unconformably on the Cambrian Amber Formation (Stauffer, 1968). Locally a conglomerate occurs at the base, followed by a grey to pinkish cross-bedded and ripplemarked felspathic quartzite with minor shale. On the basis of worm burrows and Cruziana rugosa, an Early to Middle Ordovician age has been assigned to the Misri Banda Quartzite, but because C. rugosa is a relatively nondescript form, caution is needed regarding the precise age of that unit. In another section, upper and lower contacts of the white to grey quartzite of the Hisartang Formation with the Darwaza Formation and the Inzar Limestone are conformable. This sequence also contains only trace fossils. The Panjpir Formation in another section commences with a conglomerate succeeded by slate and phyllite with quartzite and limestone intercalations. Pogue and Hussain (1986) assigned a Middle Ordovician age to its basal part. ii) Kashmir: Ordovician rocks (Hallamulla Formation and Rishkobal Group) lie over the Cambrian sequence with a sharp contact. The Hallamulla Formation has a purple, mainly matrix-supported conglomerate with flattened or stretched pebbles and cobbles of shale and siltstone. Purple conglomerate and sandstone resting over the Cambrian and below the ‘Muth Formation’ are also known from the Gabdori ravines, west of Shams Abari (Wadia, 1935). The conglomerate is followed by grey, purple shale or slate and crossbedded sandstone with sporadic conglomerate lenses. Towards its upper part, the sandstone is grey brown to rusty brown. Crinoid fragments and brachiopods long ago identified as species of Orthis and Leptelloidea (?) sp. are in need of restudy before modern generic allocations can be made and before reasonably precise stratigraphic significance can be attached to them. But, since this sequence rests on rocks of Cambrian age and underlies rocks of broadly Silurian age, the Hallamulla Formation is referred to the Ordovician; it could be Early Ordovician. iii) Spiti-Zanskar: The conglomerate of the Thango Formation (Sanugba Group) rests with angular unconformity on the Cambrian Kunzam La Formation (Bhargava and Bassi, 1998). There are two levels of conglomerate with an intervening quartzite sequence. Diamictons in the conglomerate are



from the Eocambrian-Cambrian (Batal-Kunzam La formations), the lowgrade metamorphic part of the Vaikrita, and granite (Srikantia et al. 1977). The upper conglomerate, in addition to these, also contains pebbles and cobbles of red Thango quartzarenite. The sequence above it consists of cross-bedded quartzarenite, locally showing tidal bundles, current crescents and mud-cracks. Towards its upper part are shale partings. The Early Ordovician ichnofossil Phycodes circinatum and the marine alga Prismocorollina sp. (Sinha and Misra, 2006) as well as bryozoan fragments (communication S.K.Tangri) have been found in the basal part of the Thango Formation (Bhargava, 2008). The Thango Formation is thus assigned an Early Ordovician age. The Takche Formation, from which the Ashgill (Late Ordovician) conodonts Amorphognathus superbus, A. ordovicicus, Milaculum, Icriodella cf. praecox, Panderodus sp. have been recorded (Suttner, 2003), rest conformably over the Thango Formation. From a younger level 29 species of bryozoa are known; these include eight new species—Trematopora minima, Ulrichostylus bhargavai, Ptilodictya exiliformis, Phaenopora ordinarius, Oanduellina himalayaica, Pesnastylus? vesiculosum, Ralfina? originalis and Pinocladia triangulata. Coral: Palaeofavosites (Suttner and Ernst, 2007). The most common coral is Palaeofavosites. Coral-bryozoan-algal buildups appear in the Ashgill portion of the sequence (Bhargava and Bassi, 1986). iv) Kinnaur-Uttarkhand: In this synclinorium as well, sedimentation after the Cambrian break begins with a conglomerate (Ralam Formation; Srivastava et al. 1996). It is succeeded by a sequence of quartzarenite. In Kinnaur part the Early Ordovician sequence is referred as the Thango Formation; it has produced a diverse assemblage of trace fossils (Bhargava et al. 1984). The upper part of the Ralam Formation has shale partings. It is overlain conformably by the Garbyang Formation composed of ferruginous sandy carbonate with black shale partings with small ripple bedding, herringbone cross-bedding and bands of barite. It in turn is succeeded by dolomite, marl and gypseous shale representing an evaporatic facies (S. Kumar et al., 1977); the remaining sequence consists mainly of alternating sandstone and shale with carbonate beds in its upper part (Garbyang F of S. Kumar et al., 1977). Only flat gastropods and crinoid fragments are found in the Garbyang Formation. The succeeding Shiala Formation is defined at its base by the first appearance of crinoidal limestone. It is divided into seven members, the A to G members of S. Kumar et al. (1977). The sequence from A to D consists of shale and sandstone, carbonate mud, sand units, sandy limestone, shell conglomerate, coquinoid limestone, and quartzarenite with



shelly limestone bands. Ripple bedding, parallel bedding with low-angled truncation, ripple marks, convolute bedding, penecontemporaneous deformational structures and mud pebbles characterise this sequence. Shiala D has abundant Rafinesquina. Though the exact stratigraphic level is not specified, the Ordovician nautiloids Lambeoceras sp., and Ormoceras sp. are known from this formation (G. Kumar et al., 1972). The Shiala E to G sequence consists of hard shell limestone alternating with thin to thick calcareous shale and, locally in F, sandy beds showing large-scale crossbedding. In Shiala F, penecontemporaneous deformation structures and mud pebble horizons are locally present. This level may be correlated with the level having penecontemporaneous deformation structures in the Takche Formation of the Pin and Parahio sections. The extensive development of soft sediment deformational structures at the same level is interpreted as a reflection of palaeoseismites. Part of the overlying Variegated Formation (Variegated A of S. Kumar et al., 1977) consists of nodular limestone with corals, bryozoans and stromatoporoids; it is regarded as Ordovician age (Yong Limestone of Shah and Sinha, 1974). The Yong Limestone also contains chitinozoans and scolecodonts (Khanna et al. 1985) of Late Ordovician to Early Silurian (Caradoc to Llandovery?). In Kinnaur, this sequence has been referred to as the Takche Formation; it has coral-algal buildups (Bhargava and Bassi, 1986). v) Nepal: The Damgad Formation commences with a prominent conglomerate at its base. Upward it is succeeded by a thick sequence of quartzarenite. vi) Bhutan: The Wachi La Formation in the Wachi La section rests over the mid-Late Cambrian Quartzite Formation. In other sections it rests variously over the older Deshichiling, Maneting and Quartzite formations signifying a regional unconformity. The basal part of the Wachi La Formation was found to be reefal made up exclusively of crinoids and the tabulate coral Propora of Late Ordovician to Silurian age. Since environments conducive to reef building existed in the Late Ordovician and Early Silurian in the Tethyan region, the lower-age limit of the Wachi La Formation may be specified as Late Ordovician (Bhargava, 2008).

Basin Configuration, tectonics and palaeoclimate The Early Ordovician marine basin extended from Peshawar to Nepal; the Bhutan part may have remained a positive area during that period



(Fig 10). Conglomerate forms the base of the Ordovician sequences in all areas with the exception of Bhutan. The extensive development of conglomerate is taken to indicate the presence of mighty rivers flowing down steep slopes of a sharply-risen source area (Fig. 11) suggested to have been the Kurgiakh Range. It was a sort of a foreland basin generated by the Late Cambrian Kurgiakh Orogeny, and the site of deposition of conglomerate and sandstone. The counterpart of this basin south of the Kurgiakh Range (on the

Fig. 10. Cartoon to show configuration of Early Ordovician basin

Fig. 11. Cartoon showing profile during Early Ordovician North position of the Dhargad thrust sheet of Late Cambrian age.



Lesser Himalayan side) remained a positive area until the Permian without potential for sediment- preservation. Uplift of the Kurgiakh Range was episodic. As mentioned earlier, the initial uplift was late-Early to early-Middle Cambrian (~513 My) when the submarine ridge became subareal and contributed zircons to the Lesser and Tethyan Himalayan basins. Thrusting of the Dharagad-Simla sequence over the Chilar Formation (Bhargava, 2004) led to sharp uplift of the Kurgiakh Range. Contribution of clasts to the basal Ordovician was connected with the second pulse (a major Late Cambrian one) in the Kurgiakh Orogeny. The third pulse resulted in raising part of the Early Ordovician basin; this brought about deposition of the upper conglomerate bands with clasts of the Thango Quartzarenite. The forth pulse generated palaeoseismites during the Ashgill; these are discriminable in the Pin-Spiti-Kumaon areas. Derivation of Ordovician sediments from the Kurgiakh Range. located south of the basin as indicated by the palaeocurrents in the Ordovician sediments of the Spiti-Zanskar basin (Bhargava et al. 1991). That the Kurgiakh Range was closer to the Zanskar-Spiti-KinnaurUttarkhand-Nepal region is indicated by the presence of large clasts in those areas. Shrinkage cracks and current crescents in the Spiti-Kinnaur succession (Bhargava and Bassi, 1998) also accords with those areas being nearer the coastline. North-westwards, the ridge swerved away from the basin; this is reflected by smaller clasts in the conglomerates of the Peshawar region and Kashmir where the thickness of conglomerates is also much less. Coarser material brought down by these rivers would, conceivably, have been reworked on wide beaches. Similar to Cambrian antecedent rivers continued to flow across the Kugiakh range contributing detritus from distant sources (Myrow et al. 2010). Overall, the Early Ordovician basin was shallow marine, characterised by high-energy. The extensive red-coloured sediments accord with warm climates and rivers having enormous carrying capacity, as does the transport of huge amounts of coarser clastics. By the Late Ordovician (Ashgill), the Kurgiakh Range would have been much eroded, with the sea invading the Bhutan region. The climate appears to have become somewhat arid as a result of little detritus reaching the basin. The stage was set for carbonate sedimentation. Warm climate promoted evaporite facies (S. Kumar et al., 1977) and reef growth (Bhargava and Bassi, 1986). There could have been intervening pluvial periods, temporarily resulting in delivery of detritus to the basin and arresting of reef growth.



SILURIAN Characteristic fossils for defining the Ordovician-Silurian boundary have not been found in the Himalaya. Their absence could mean a brief sedimentologic break in response to the Hiranantian glaciation when it produced a global fall in sea level; it could also reflect lack of intense investigations. In all the sections, Ordovician sequences appear to grade into the Silurian up-sequence.

Tethyan Himalaya i) Peshawar Basin: The Kandar Formation exposed east and south-east of Peshawar consists of argillite, metasiltstone, calcareous quartzite and impersistent conglomerate. Llandovery-Wenlock faunal elements are known from the basal part of the Kandhar Formation (Pogue et al., 1992). Reports of the conodonts Kockelella variabilis, Polygnathoides siluricus, Ozarkodina remscheidensis eosteinhornensis and O. crispa accord with a LudlowPridoli interval (Talent and Bhargava, 2003). Ozarkodina remscheidensis eosteinhornensis and the long-ranging O. excavata excavata and Panderodus unicostatus were reported by Barnett et al. (1966) from the youngest horizon of the Kandhar Formation and from the Nowshera Limestone. Hussain et al. (1990) reported bivalves, cephalopods and conodonts of Ludlow age from limestone in the basal part of the Panjpir Formation. Crinoidal limestone in the upper part of the Panjpir has produced indubitably Pridoli conodonts, as have horizons low in the Nowshera Limestone (Talent and Mawson, 1979); the latter unit has now been demonstrated to continue without discernible stratigraphic break to at least as young as late Emsian at Pir Sabak in the Nowshera area (Mawson et al., 2003). ii) Kashmir: The Kashmir Silurian is represented by white, grey and locally purplish cross-bedded quartzite, calcareous shale, sandstone and hard limestone (Gugaldhar Formation). Fossils from this formation have been reported as Lindstroemia cf. L. bina, Alveolites sp., several species of Orthis, Leptaena rhomboidalis, several species of Strophomena, Atrypa sp., Orthoceras, Cyrtoceras, Calymene, Acidaspis, Illaenus sp., Triplecia insularis and Plectambonites sp. but the identifications are very much out of date. Reed (1912) assigned this fauna to the Llandovery, whereas Boucot and Gauri (1968) suggested an Ashgill-Llandovery slot—an age similar to the equivalent formation in the Spiti region deduced from conodonts. The hard limestones represent small coral-algal buildups.



iii) Spiti-Zanskar: A sequence of dolostone, shale and siltstone forming the upper part of the Takche Formation is suggested to be Silurian. The basal and middle parts of this formation are believed to be Ordovician. In Zanskar, this formation is more arenaceous and either not exposed or has limited development due to pre-Muth erosion. The carbonate content increases eastward with maximum carbonate content in the Pin Valley. The sequence includes small coral-stromatoporoid-bryozoan-algal buildups (Bhargava and Bassi, 1986). Llandovery conodonts are known from its upper part (Eric Draganits, unpub. data); the overlying sequence is a stromatoporoid buildup. This part may be Wenlock as its equivalent horizon in Kinnaur has produced a coral, Radiastraea sp. (Bhargava and Bassi, 1998). iv) Kinnaur-Uttarkhand: The upper part of the Yong Formation may be Llandovery (Khanna et al., 1985) and, if so, the overlying Variegated B and C (of S. Kumar et al., 1977) are likely to be still younger. The Muth B (of S. Kumar et al., 1977) has silicified bryozoans, corals and bivalves; it thus appears to be part of the Variegated Formation; it is not typical Muth (Bhargava, 2008a). This sequence may extend into the Wenlock. v) Nepal: The Dark Band Formation, exposed on the left bank of the Kali Gandaki River is assigned a Silurian age (Talent and Bhargava, 2003). From this formation, along the northern flank of the Nilgiri massif, Bodenhausen (1964) reported early Late Llandovery graptolites. The Chitlang Formation in the Chandragiri-Phulchauki Synclinorium has produced Llandovery conodonts as well as trilobites of Wenlock age; the overlying Chandragiri-Phulchauki limestones must therefore be regarded as perhaps broadly Wenlock-Ludlow but the Phulchauki limestones are certainly not the source of latest Givetianearly Frasnian conodonts reported from them (see Talent et al., 1988, 1991; Talent 1989; Webster et al., 1993). The now poorly preserved corals and stromatoporoids in the Chandragiri Limestone may have formed small buildups, but deformation has hampered interpretation. vi) Bhutan: Limestone forming the upper part of Wachi La Formation may be Silurian.

Basin Configuration, tectonics and palaeoclimate The fossil record in most areas, apart from the Peshawar region, seems to end in the Llandovery, though extension into the Wenlock cannot be excluded. In the Peshawar region, specifically about Nowshera, as



mentioned above, the Silurian sequence continues up from the Pridoli into the Emsian (Mawson et al., 2003). In the Tethyan Himalaya, Silurian thicknesses gradually increase from west to east concurrent with increase in carbonate. The decrease in thickness may be due to uneven pre-Muth erosion, whereas the increase in carbonate content may indicate relative deepening of the basin eastwards. The Nilgiri Massif in Nepal may have formed the distal part of the basin until at least, according to grapolites in Late Llandovery, as revealed by the graptolite fauna. In the Tethyan regions, several shoaling cycles commence from midlower shoreface and terminate in upper shoreface sedimentation close to an undathem (microfacies belt 7 of Wilson, 1975) or the subtidal-intertidal interface with periodic storms (Bhargava, 2008b) or zero energy carbonate sedimentation (S. Kumar et al., 1977). Initially (Llandovery) the Peshawar Basin was shallow, witnessing deposition of clastics with occasional conglomerate. There was a deepening in Ludlow-Pridoli time (Talent and Bhargava, 2003) creating conditions favourable for conodont animals to thrive; the terminal part of the sequence displays a regressive cycle leading to withdrawal of the sea. An interval of erosion occurred prior to the ? EarlyMiddle Devonian transgression (Bhargava, 2008b). It is difficult to decide whether the movement terminating the Silurian basin was the culmination of the Cambrian orogeny or was an unrelated and independent event. A preponderance of limestone indicates extension of a warm, possibly arid climate, with periodic humid phases when clastics were deposited.

ACKNOWLEDGEMENT Grateful thanks are due to Prof J.A. Talent, who was kind enough to critically review this manuscript despite his illness. The paper formed the basic material for the 40th Birbal Sahni Memorial Lecture delivered on the Founder’s Day Function of the Birbal Sahni Institute of Palaeobotany, Lucknow on 14th November, 2010.

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