carbon isotope and sequence stratigraphy

Geodinamica Acta (Paris) 1996,9,2,57-77

Late Permian and Early Triassic evolution of the Northern Indian margin: carbon isotope and sequence stratigraphy Evolution de la marge indienne du Permien superieur au Trias inferieur: stratigraphie isotopique (carbone) et sequentielle Aymon BAUD*, Viorel ATUDOREI * and Zachary SHARP **

ABSTRACT. - The Northern part of Great-India underwent an early rifting phase in the late Paleozoic, just at the end of the large scale Gondwanian glaciation. The beginning of the rifting processes is marked by large hiatus and discontinuities (paraconformities) between the early or middle Paleozoic sedimentary succession and the discontinuous middle-late Permian Traps and transgressive sediments. The Northern Indian passive margin consists of the present High and Lower Himalaya and a small part of the Indian craton and their sedimentary cover. The Permian rift shoulder is located in the Higher Himalaya, with part being in the underthrusted Lower Himalaya. The rim basin Oandward of the shoulder) is well developed in the Pottawar Salt Range area. From the rifting to the beginning of the drifting stages (early late Permian to late early Triassic time), the sedimentary evolution is characterised by three transgressiveregressive (T-R) second order cycles, two in the late Permian and one in the early Triassic. The break-up of the rift occurred during the second cycle (late Dzhulfian). In the Salt Range area, these three T-R cycles have been subdivided in eight third order sequences, five sequences for the upper Permian and three for the lower Triassic. At the end of Permian, hiatuses, gaps and local erosion of part of the margin are direct consequences of a first order relative sea-level fall; this is also the time of the largest extinction event of the Phanerozoic that deeply affected the carbonate productivity and the stratal patterns. With the following worldwide sea-level rise, a rapid and large scale transgression occurred in the early Triassic, well dated and recorded on the whole margin. High rate thermal subsidence gave way to generalized pelagic deposits about 2 My after the transgression. Profiles of whole rock inorganic carbon and oxygen isotopes from Guryul Ravine and Palgham sections in Kashmir, Nammal

* Musee de Geologie, UNIL-BFSH2, CH-1015 Lausanne, Switzerland. e-mail: [email protected]. ** Institut de Mineralogie, UNIL-BFSH2, CH-1015 Lausanne, Switzerland. -

Gorge and Landu sections in Trans Indus Ranges (Pakistan), Thini Chu section in Kali Gandaki Valley, Central Nepal are presented in connection with the sequence stratigraphic analysis. The upper Permian record of high positive a13c values are closely correlated with the second order T - R cycles and the third order sequences. The results presented in this study confirm the drastic drop of a13c from the high positive values that characterised the upper Permian to lower values in the lower Triassic time. Stratigraphic correlation problems in the lower Triassic using carbon isotope geochemistry are briefly discussed. A positive a13c excursion of 4-5%0 near the Smithian - Spathian substages boundary is observed for the first time. The alSO values of samples from all the sections display major variations suggesting that the oxygen isotope record has been significantly affected by meteoric diagenesis, deep burial diagenesis orland monsoon signature. Key-words: Carbonate Platform, Himalaya, Kashmir, Rifting, Salt Rang, Stable Isotope. RESUME. - C'est Ii. la fin du Paleozoique, peu apres les grandes glaciations gondwaniennes que la partie septentrionale de la grande plaque indienne fut soumise Ii. des processus de fracturation et de riftogenese. Le debut de ces processus se marque par des lacunes etlou des discordances entre les successions sedimentaires du Paleozoique inferieur et moyen et celles, discontinues, accompagnees localement de basaltes de plateau du Permien moyen et superieur. Les differents elements de la marge passive Nord-indienne se trouvent actuellement dans les unites du Haut- et du Bas-Himalaya et leur couverture sedimentaire ainsi que, partiellement, sur Ie craton indien adjacent. L'epaulement du rift permien se trouve en partie dans les unites du haut-Himalaya et en partie cache dans Ie sous-charriage du Bas-Himalaya. Un des bassins peripheriques est bien developpe dans la region occidentale de Pottawar et des Salt-Ranges (Pakistan). De la riftogenese jusqu' Ii. la croissance oceanique (debut du Permien superieur jusqu'l\ la fin du Trias inferieur), l' evolution sedimentaire est caracterisee par trois cycles trans-

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A. BAUD, V. ATUDOREI, Z. SHARP

gressifs - regressifs de deuxieme ordre ( Cycles T-R), deux dans Ie Permien superieur et un dans Ie Trias inferieur. La cassure (break-up) et l'ouverture oceanique par cisaillement simple s'est produite it la fin du Permien, au debut du deuxieme cycle T-R (Dzhulfien). Dans les profils de la region des Salt-Ranges, ces trois cycles T-R ont ete subdivises en huit sequences de troisieme ordre, cinq dans Ie Permien superieur et trois dans Ie Trias infmeur. A la fin du Permien, des lacunes et des erosions locales sont la consequence directe d'un abaissement de premier ordre du niveau marin relatif. C'est aussi Ie temps de l'extinction la plus considerable de tout Ie Phanerozoique, extinction qui a affecte de maniere durable la productivite des carbonates, la disposition et la geometrie des depOts marins. C'est une transgression !res rapide et it l'echelle mondiale qui a suivi ces evenements. Elle est bien datee ici du debut du Trias (Induan inferieur) et enregistree sur toute la marge. Une forte subsidence thermique se marque deux millions d'annees plus tard par une generalisation des depOts pelagiques. Nous presentons ci-apres en relation avec l'analyse sequentielle, les profils isotopiques du carbone (auC) et de l'oxygene (a I80) traites sur les carbonates de roche totale des coupes lithologiques suivantes : Guryul Ravine et Palgham au Cachemire (lnde), Gorge de Nammal et de Landu dans les Salt-Ranges (Pakistan) et Thini dans la vallee de Kali Gandaki (Nepal central). Les couches du Permien superieur montrent des valeurs de auc largement positives et qui fluctuent en fonction des systemes de depots (transgression-regression). Entre les couches regressives de la fin du Permien et la transgression basale du Trias, notre etude confirme la chute impressionnante de auc depuis des valeurs fortement positives jusqu'it des valeurs tres negatives. L'utilisation de ce nouvel outil pour les correlations it la limite Permien-Trias est egalement discutee avec l' aide de la biozonation basee sur les conodontes et en comparaison de la coupe de reference de Gartnerkofel-1. Vers Ie haut du Trias inferieur (limite Smithien-Spathien), une excursion positive de auc est observee pour la premiere fois. En ce qui conceme les valeurs isotopiques de l'oxygene alSO, elles sont systematiquement fortement negatives avec d'importantes fluctuations indiquant les effets d'une diagenese prononcee. Celle-ci peut etre, dans certains cas, d'origine meteorique avec influence ou non de type mousson, ou-dans d'autres cas (profils provenant de la region himalayenne) d'une origine par enfouissement ou par faible metamorphisme regional. Mots-des : Cachemire, Himalaya, Isotope stable, Plateforme carbonatee, Rifting, Salt Range.

I. INTRODUCTION Localities presented in this paper come from three areas of the Northern Indian margin: Salt Ranges, Kashmir and Central Nepal (fig. 1). All our data are plotted according to the stratigraphic stages and biozones of fig. 2. Discussions are still in progress for a united late Permian time scale and for a Permian-Triassic global boundary stratotype section and point (GSSP). For each IOFality, controversial correlations are briefly discussed. Existing problems are the definition of the lower boundaries of the Midian, the Dzhulfian and the Induan Stages. Concerning the stratigraphical cycles and sequences, we are using the hierarchy (order) defined by Vail et al. (1991).

-

Fig. 1. - Location map of the studied sections. 1. Salt Range and Surghar Range sections. 2. Kashmir sections. 3. Thakkhola section. Fig. 1. - Carte de situation des profils eludiis. 1. Profils des Salt Range et Surghar Range. 2. Profils du Cachemire. 3. Profil de la Takkhola.

Recent paleogeographic maps and description of the late Murgabian and late Anisian paleoenvironments of the Northern Indian margin are given in the Tethys Atlas by Baud et al. (1993) and by Marcoux et al. (1993). From the geodynamical point of view, the northern part of the Great-India was subjected to an early rifting phase in the late Paleozoic, just at the end of the large scale Gondwanian glaciation. The beginning of the rifting process is marked by large hiatuses and discontinuities (paraconformities) between the early or middle Paleozoic sedimentary succession and the discontinuous late Paleozoic transgressive sediments. The asymmetric rifting geometry consists of a northern Lower Plate - the present Ladakh Karakoram and Transhimalaya continental crust with their former sedimentary cover, and a southern Upper Plate - the present High and Lower Himalaya and small part of the Indian craton and its sedimentary cover. A sketch map of the geodynamic evolution of an asymmetric rift and a figure showing the different part of the N Indian Permian rift are given in Stampfli et al. (1991, fig 1 and 5). If the Lower Plate evolved into an active margin during the late Mesozoic, the Upper Plate corresponds to the future Mesozoic - early Cenozoic Indian passive margin. From the Indian craton to the rift proper, the rift geometry is hidden in the presently largely deformed underthrusted overthrusted Indian margin. The rim basin (landward of the shoulder) is well developed in the Pottawar - Salt Range area. The rift shoulder is found in the Pir Panjal and High Himalayan Ranges, and part of it should be found in the underthrusted Lower Himalaya. The blocks facing the central part of the rift are represented now in the Zanskar - Spiti sedimentary belt and elements of the highstand blocks that appeared during the earliest drifting stage have been found in the exotics of the Indus -

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LATE PERMIAN AND EARLY TRIASSIC EVOLUTION OF TIlE NORTIlERN INDIAN MARGIN

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Tirolites Anasibirites Prionoiobus Gyronites'

N. waageni N. pakistanensis N. cristagalli N. dieneri

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Vedioceras

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N. siciliensis N. serrata

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Fig. 2. - Stratigraphic stages and biozones for the upper Permian and lower Triassic. Tethyan stage names after Ross et al. (1994). Ammonoids and conodonts after Yang & Li (1992) and Matsuda (1985) modified. For the T-R cycles, see description in the text. Magnetostratigraphy from Haag & Heller (1991). Fig. 2. - Elages et biozones du Permien superieur et du Trias inferieur. Etages rethysiens d'apres Ross et aI. (1994). Biozones d'ammonoiaes et conodontes suivant Yang & Li (1992) et Matsuda (1985), modifie. Les cycles T-R sont decrits dans Ie texte. La magnetostratigraphie est d'apres Haag & Heller (1991).

Yarlung suture zone or in the allochthonous coloured melange, for example in the Spongtang klippe area (Reuber & Colchen, 1987). From rifting to early drifting stages (early Late Permian to early Triassic time), the geodynamic and sedimentary evolutions are characterised by at least seven different events (Baud et ai., 1989b). 1. A laterally extensive extrusion of "plateau basalts" (over 100 000 km2), the Panjal Traps, mainly on the broad rift shoulder during the Kubergandian - early Murgabian times. 2. A huge carbonate platform (the Wargal Formation) transgressing in the rim basin during late Murgabian - early Midian time. The first upper Permian transgressive - regressive cycle (T-R or second order cycle) is recorded in the growth and demise of this carbonate platform (Midian time). 3. A block tilting and uplift phase with erosion processes occurring during the early Dzhulfian. A sudden terrigenous influx occurs both on the rift side and on the land side and marks the boundary between the lower and the upper T-R cycles. -

In the rim basin (Pottawar-Salt Range area), a shallow water mixed carbonate - clastic ramp, the Chhidru Formation, overlies the Wargal carbonate platform (Pakistanese-Japanese Research Group or PJRG, 1985). To the North, in the Kashmir area, a submerged part of the shoulder developed an offshore mixed clastic carbonate deltaic complex, the Zewan Formation (Nakazawa et at., 1975). The Kuling sandstone represents in Zanskar the distal part of this complex (Baud et ai., 1984) and turbiditic, flysch like deposits with volcanic clasts of Dzhulfian-Changhsingian? age appear near the present Indus suture zone in the Markha Valley (NE Zanskar, Stutz, 1988).

4. The marine submersion of the shoulder indicates the beginning of the thermal subsidence and the transition to the drifting stage (late Dzhulfian - early Changhsingian). The main consequences are a general starvation and hiatuses as we can observe in the rim basin (laminated dark silty to sandy deposits of the top of the Chhidru Formation) and on the rift side of the shoulder with phosphatic shales deposits of the upper 59-

A. BAUD, V. ATUDOREI, Z. SHARP

Kuling Formation (Gaetani et al., 1990; Garzanti et al., 1994c; Nicora et al., 1984). As shown by the study of large exotic blocks (Reuber & Colchen, 1987; Bassoullet et al., 1978) highly microfossiliferous lime packstones (Colaniella limestones) grew within trachytic volcano - sedimentary deposits during the youngest Permian (Changhsingian), on uplifted compartments of the intermediate (and oceanic?) crust. 5. Hiatuses, gaps and local erosion in part of the margin are direct consequences of the worldwide, first order, end of Permian sea-level fall (Holser & Magaritz, 1987); this is the time of the largest extinction phenomenon of the Phanerozoic (see also discussion in Erwin, 1993). 6. With the following worldwide sea-level rise and transgression, an important change in sedimentation occurs at the Triassic dawn. In the Salt Range, high energy dolomitic grainstones with glauconite (middle Kathwai Member) transgress over dolomites of the lower Kathwai Member or locally, directly on the starved uppermost Chhidru deposits. On the former shoulder (Kashmir), deepening marine conditions are indicated by open marine shales with ammonoids limestone lenses (Otoceras beds of the Khunamuh Formation) and fine graded silts of distal pelagites transgressing over lowstand, shallow Claraia shales and limestones. Seaward, very condensed cephalopods limestones occur (basal Lilang Group of Zanskar, Nicora et al., 1984). The Lamayuru exotic (Bassoullet et al., 1978) is characterised by a manganese crust deposit on the Changhsingian Colaniella limestone. 7. A generalisation of the pelagic limestone facies is recorded about 2Ma later at the transition between early and late Induan time with the cephalopods rich Lower Ceratite limestone deposition in the proximal part of the margin (Salt Range) and thin, condensed limestones and shales in the distal part (Tamba Kurkur Formation of Zanskar-Spiti and Nepal Himalaya). Some of the exotics record the earliest "Hallstatt" type limestone deposits directly on the Mn crust.

II. STABLE ISOTOPE INVESTIGATIONS AND PREVIOUS WORK A number of carbon isotope studies of the Permianffriassic boundary in various areas of the world have been recently carried out (Holser & Magaritz, 1987; Oberhansli et aI., 1989; Baud et al., 1989a; Malkowski et aI., 1989; Chen et al., 1991; Magaritz et al., 1992). A first study on Permian-Triassic sections of the North Indian margin of Gondwana is given in Baud et al., (1989a). The potential of carbon isotope studies as a stratigraphic tool for Permian-Triassic sequences has been emphasized by these studies. However, correlations are often difficult in the absence of biostratigraphical constraints. In terms of biozonation and systematic -

paleontology, the northern Indian margin has been well studied for more than a century and two sections given in this paper (Nammal Gorge, Guryul Ravine) are reference sections for the lower Triassic.

Methods and results Profiles of whole rock inorganic carbon and oxygen isotopes were studied in marine carbonates from the Guryul Ravine and the Palgham sections in Kashmir, the Nammal Gorge and the Landu sections in the Salt Range and the Surghar Range (Pakistan), the Thini Chu section in the Kali Gandaki Valley, Central Nepal. Carbon and oxygen isotope analyses were performed using standard techniques (McCrea, 1950), in the Isotope Laboratory of the Lausanne Institute of Mineralogy. Thin section examination eliminated samples that had suffered coarse or extensive recrystallisation, which could indicate reequilibration of primary marine values. 5-10 mg samples of powdered carbonate were drilled from polished rock chips using a dental burr. Each sample was individually reacted with 100% phosphoric acid at 50°C, then the evolved CO 2 was cryogenically purified and the isotopes were measured with a Finnigan Mat 251 mass-spectrometer. The working standard in the Lausanne Laboratory is calibrated relative to NBS-19. All ~13C and ~180 values are reported relative to PDB. For NBS-19, ~13C is 1.95%0 and ~180 is -2.20%0 (Coplen et al., 1983). Analyses were performed in duplicate with reproducibility better than 0.1%0 for carbon and 0.15%0 for oxygen isotopes. The results of stable isotope analyses are listed in Table I. An important problem for carbon isotope studies is whether the measured ~13C values principally reflect the isotopic composition of the seawater from which the carbonate was precipitated, diagenetic inputs, or metamorphism. We assume that the measured ~13C values roughly approximate the depositional composition. There are some doubts for the Guryul Ravine section which experienced a severe thermal metamorphism, the ~13C values being generally depleted compared with other sections. However, even in this section the shape of the ~13C curve is similar with published curves for the same time span. Most dolomites from our sections are isotopically indistinguishable from associated limestones. However, it is known that dolomitization has little effect on carbon isotope composition, a shift of 1%0 occurring only in extreme cases (Land, 1992).

III. SALT RANGES The upper Permian to lower Triassic succession is well exposed in gorges that dissect the Salt Ranges - TransIndus Ranges of Northern Pakistan (fig. 3). The main studies on this area have been summarized in Kummel and Teichert (1970) and part of the recent literature in Wignall & Hallam (1993). 60-

LATE PERMIAN AND EARLY TRIASSIC EVOLUTION OF THE NORTHERN INDIAN MARGIN

Table I. - Carbon and oxygen isotope analyses of carbonates from Nammal Gorge, Landu, Guryul Ravine, Palgham and Thini Chu sections. Reference level for the metric scale: base of Kathwai Member for the Nammal Gorge and Landu sections; base of Khuna.muh Formation for the Guryul Ravine and Palgham sections; base of Panjang Member for the Thini Chu section. All /)I3C and a180 values are reported relative to PDB. Tableau I. - Analyses isotopiques du carbone et de I'oxygene des carbonates des profits de Nammal Gorge, Landu, Guryul Ral'ine, Palgham et Thini Chu. Niveau de reference 0 pour l'echelle metrique : base du Membre de Kathwai dans les coupes de Nammal Gorge et de Landu; base de La Formation de Khunamuh pour les coupes de Guryul Ravine et de Palgham; base du Membre de Panjang pour la section de Thini Chu. Toutes Les valeurs {jue and {jI80 sont relatives au standard PDB. SAMPL, I I Depth No. S"C SilO ( m ) Nammal 4.61 -6.45 -163 W 214 W208 4.77 -6.65 -161 W 189 4.23 -7.44 -138 W 175 5.29 -9.18 -132 W162 5.27 -8.14 -128 W 151 5.01 -6.94 -120 W 146 5.07 -7.53 -113 W 134 5.41 -8.23 -99 W 126 5.27 -8.56 -92.5 W 118 5.03 -7.98 -84 W 113 4.71 -8.26 -79 W 108 4.63 -8.03 -74.5 W 103 4.55 -6.96 -71 4.43 -8.14 W98 -68 -7.22 W92 4.21 -63 WOO 3.72 -3.33 -56 3.67 W 72 -5.78 -49.5 CH65 3.43 -3.23 -38.5 3.47 -5.61 -37.5 CH63 CH62 3.66 -4.51 -36 3.77 -5.79 -34.5 CHao CH59 3.85 -5.31 -33 3.68 -6.7 -31.5 CH57 CH53 3.78 -5.67 -27.5 -7.83 CH49 3.06 -25 3.64 -8.99 -20.5 CH43 CH42 3.64 -7.44 - 19 CH37 3.21 -7.68 -17 CH36 2.17 -8.87 -15 3.09 -7.54 CH35 -14 3.05 -7.35 -12 CH33 CH25 2.33 -8.9 -7.5 CH 13 1.47 -9.55 -2.5 -0.24 -5.5 N3a1 0.3 -0.28 -5.08 0.32 N3a1' N3a1" -0.21 -5.84 0.35 -0.19 -5.61 N3a1'" 0.4 -4.33 N3c 1.6 1.2 N4a 1.57 -6.11 1.9 N5b 1.22 -6.41 2.7 0.28 -9.41 N6a1 3.3 N6b1 0.12 -8.99 3.8

SAMPLEI No. N6b4 N6e2 N6e3 N7a N7b N7e NBa N8b Nac N8d

N8e N8f Nag N81 N16a1 N16a2 N16e1 N16c3 N17a5 N17b3 N17e1 N18a1 N18a3 N18b1 N19a1 N19a2 N19b3 N1ge1 N20a1 N20a2 N20b1 N20b4 N20e2 N21a3 N21b1 N21b2 N21b3 N22b N26e N26d N27a2 N27a5 N27e3

snc

0.57 0.3 0.22 -0.12 -0.61 0.09 -0.44 0.66 0.18 1.04 0.72 0.98 0.78 1.53 -0.64 -1.8 -1.77 -1.66 -1.99 -1.89 -1.95 -2.3 -2.32 -2.24 -2.16 -2.18 -2.06 -0.6 0.59 0.45 0.68 1.19 1.84 1.74 -0.82 0.48 -0.46 -1.38 -2.13 -1.8 -1 ;72 -1.89 -1.66

I 11110 I ( -7.32 -6.22 -6.06 -5.3 -7.65 -5.34 -7.58 -5.31 -4.29 -5.16 -5.56 -5.18 -5.17 -5.57 -8.11 -8.43 -8.97 -8.69 -8.36 -7.91 -7.61 -7.54 -8.55 -8.62 -8.18 -8.31 -8.56 -7.52 -6.97 -8.62 -6.66 -6.53 -7.25 -9.18 -6.94 -8.81 -6.27 -7.39 -7.47 -7.22 -7.49 -7.54 -3.61

Depth m ) 4 4.2 4.3 4.6 5.1 5.7 6.3 7.2 7.8 8.8 10 10.9 12.3 14 49 52.4 57.5 57.9 61 62.3 62.8 63.4 64.5 64.8 66.6 67.2 69 69.4 69.9 70.3 70.9 71.4 72.2 73.7 74.4 75.1 75.8 82 104.2 104.8 107 110 118.5

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Depth SAMPL, No. II n C 11"0 m ) GU19 -2.94 - 11 GU20 -3.09 -10.8 22 25 GU21 -2.89 -10.9 -2.94 -11.1 29 GU22 32 GU23 -2.49 -10.8 -3.38 -10.3 33.5 GU24 36.5 GU25 -3.78 -10.3 -10.6 38 GU26 -2.5 39 GU27 -1.36 -10.5 39.5 GU28 -1.69 -9.62 Palgham 41.5 -11.8 42.5 P1 1.92 1.54 -12.1 43.5 P2 -11.9 47 P3 0.63 1.14 -11.5 47.5 P4 -12 48.5 -2.07 P5 -2.63 -11.6 49.5 P6 51.5 -3.04 -10.9 P7 -2.84 -10.8 P8 -4.29 -11.4 -75 P9 -73 -3.56 -10.9 P10 -72 -2.59 -11.2 P12 -71 P 11 -2.78 -10.7 Thlnl Chu -19.5 -0.15 -12.6 -14.7 T4 T5 -0.95 -9.88 - 8 -5.6 T6 -0.53 - 10 -0.9 -10.7 -2.5 T7 -0.55 -9.36 -0.5 T8 -0.05 T9 -0.77 -23.2 -0.16 -18.8 0.05 T10 0.2 -1.07 -20.7 T11 -1.34 -21.3 0.25 T15 0.4 T16 -0.3 -13.1 -0.26 -11.5 1 T17 1.5 2.3 3.4 3.9 5.2 5.8 7.1 8.0

I(

Depth m) 8.5 9.6 10.4 10.7 12.1 14.9 17.9 20.4 22.4 25.4 -13 - 8 -6 -0.3 0.1 2 8 10 17.5 22 23.5 26 0.2 0.5 0.9 1.3 1.5 1.6 1.7 1.9 2 16 17

Permian-Triassic boundary occurs between the lower and middle unit of the Kathwai Member (see fig. 4 and 6).

A stratigraphic chart is given in fig. 4, with the lithounits, sequences and T-R cycles. Our interpretation of a hiatus between the Chhidru and the Mianwali Formation has been expressed in Baud et al. (1989a), With respect to age, we now agree with Nakazawa (1993) that part of the early Griesbachian is recorded in the lower and middle Kathwai dolomite. This is also the opinion of Wignall and Hallam (1993), but for the PJRG (1985), the -

I

SAMPL, No. II n C 111 '0 Landu L7a 0.79 -7.6 LBa 0.83 -8.32 L8b 0.66 -8.49 L9a2 -1.43 -8.78 L9b3 -1.55 -8.66 L10b3 -2.11 -8.26 L10e1 -1.99 -8.6 L10c3 -1.92 -8.46 L10e4 -2.17 -8.23 l11a -1.42 -7.98 L11b -1.58 -8.32 l11e -1.36 -8.73 l13a 4.74 -4.79 l13b 3.75 -4.68 l14a 3.41 -7.04 L14b1 3.47 -6.22 l14 b3 2.3 -7.84 Guryul Ravine GU44 2.44 -11.3 GU42 2.95 -11.1 GU41 2.69 -10.9 GU40 1.63 -10.4 GU35 3.13 -12.1 GU34 1.27 -11.3 GU32 1.7 -10.6 GU31 2.28 -11 GU30 -0.59 -10.8 GU29 -0.31 -10.1 GU1 -0.93 -9.93 GU2 -0.4 -9.79 GU4 1.14 -10.9 GUS -10.2 - 1 GU8 -0.85 - 1 1 GU9 -0.14 -10.2 GU10 0.04 -11.2 GU11 -1.23 -11.1 GU48 -1.05 -11.3 GU13 -1.12 -11.1 GU15 -1.61 -1 1 GU16 -2.32 -11.5 GU17 -2.5.7 -11.6 GU18 -3.24 -11.2

A. Nammal Gorge section (fig. 5) A preliminary Narnmal Gorge carbon isotope profile has been published in Baud et al. (1989a), based on analyses performed in Rehovot Laboratory. We present

61

A. BAUD, V. ATUDOREI, Z. SHARP

For the Upper Permian deposits, our interpretation of the sequence stratigraphy presented below is mainly based on field work and discussions with B. Haq and P. Vail, and for the lower Triassic on Haq et al. (1988). The main divergence of opinion is the meaning of the Chhidru-Kathwai contact - a flooding surface with no hiatus for Haq et al. (1988) and a sequence boundary with a gap and paraconformity in our interpretation.

1) Upper Permian T-R cycle CI

Fig. 3. - Location map of the Nammal Gorge and Landu sections. Fig. 3. - Carte de situation des profils de Nammal et de Landu.

here the results of a more detailed sampling, covering the upper Permian and most of the lower Triassic. In addition, a part of the Landu Nala profile has been analysed to determine whether the variations of Sl3C are influenced by diagenetic effects. Permian and basal Triassic samples have been collected by one of the authors (A.B.) with C. Jenny and J. Marcoux during an IGCP project 199 field seminar organised by B. Haq in December 1987. Additionally, we received a set of samples which were collected for magnetostratigraphy studies from M. Haag (Zurich). Triassic samples came from field work carried out in 1974 by A. Baud, P. Bronnimann, J. Guex and L. Zaninetti (Guex, 1978).

Carbon and oxygen isotopic composition of carbonates are plotted against depth on the stratigraphic column in fig. 5. On the basis of the occurrence of Neoschwagerina margaritae in the lower part of the Wargal Formation, the carbonate shallow marine transgression over the lower Permian (Kubergandian) Amb Formation is late Murgabian - early Midian in age (late Permian). The Wargal carbonate platform is subdivided into 5 informal units (1-5) by PJRG (1985) and consists of a second order T-R cycle (CI). Based on field data, the T-R cycle can be subdivided into three shallowing upward third order sequences capped by regressive dolomite, marked SI to S3 (fig. 5). High positive BI3C values (up to 4%0) occur in the SI transgressive calcarenitic deposits. The boundary between SI and S2 is the top of the regressive dolostone of the litho-unit 3, where we see an increase of Sl3C to values of more than +5%0 in the S2. The boundary between S2 and S3 is linked with regressive dolomites in the upper part of the unit 4a and we note a slight decrease of SBC values in these highstand dolomites. The transgressive part of the sequence S3 consists of the thick bedded calcarenites of the unit 4b. We have a new

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+2%0 in the upper part of unit D. Five meters below the top, a large shift of about 3%0 of 013C to a negative value is recorded. 2) Early Triassic Only the basal part of the next C III T-R cycle has been inY'estigated. that is the third sequence (S3) belonging to the lower Triassic Khunamuh Formation. With the large transgression we note an important change in sedimentation. The lowstand deposits consist of thinly

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Fig. 10_ - Carbon and oxygen isotope profile at the Guryul Ravine section (Kashmir, India), sequences and systems tracts. LSD: lowstand deposits; TO: transgressive deposits; HSD: highstand deposits. Fig. 10. - Profils isotopiques du carbone et de I'oxygene de 10 coupe de Gmyul Ravine en relation avec les cycles T-R, les sequences

et les systemes de depl}t. LSD : cortege de bas-niveau: TD : cortege t~ansgressif; HSD : cortege de haut-niveau.

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