Palaeoenvironments of the Permian–Triassic transition sections in Kashmir, India

Abstract

Detailed measurements of two Permian–Triassic boundary sections at Guryul Ravine and Pahlgam in Kashmir indicate continuous sedimentation across the boundary, though with a marked change from shallow storm-influenced shelf to deeper shelf below wave base. Successive shell beds through the 2.5-m-thick transitional latest Permian section at Guryul formed as in situ accumulations with little sign of either erosional breaks or condensation. Faunal change takes place gradually in the late Permian transition beds, as elsewhere in the Tethyan sections. There is no major event layer at the Permian–Triassic boundary, and little evidence for dysaerobic conditions. Both the Kashmir and East Greenland Permian–Triassic sections have sedimentation rates that are orders of magnitude greater than at the condensed Meishan reference section. They show that the end-Permian extinction was associated with transgression but not with immediate onset of anoxic conditions. Macrofaunal extinction seems associated with reduction in primary production, which would also explain the dwarfism of fossils found in all Permian–Triassic sections studied so far.

Introduction

The Permian–Triassic boundary sections in Kashmir, India, have long been famous as among the most complete in the world, with apparent continuous sedimentation and gradual faunal changes across the boundary (Sheng, 1984).

During late Palaeozoic times, Kashmir formed part of Gondwanaland and lay on the southern side of the Neotethys Ocean adjacent to Oman (Fig. 1, inset). Rifting and eruption of Middle Permian basalts (Guadalupian) (Baud et al., 1996) was followed by separation of blocks off the northern edge of Gondwanaland, the formation of a Neotethys Ocean and rapid thermal subsidence of the northern Gondwana margin in the Upper Permian and Triassic (Baud et al., 1996, Bhat, 1982, Bhat, 1984, Brookfield, 1993, Gaetani et al., 1990, Stampfli et al., 1991). It is this rapid subsidence which allowed continuous sedimentation across the Permian–Triassic boundary on the northern Gondwanaland shelf when elsewhere the large end-Permian eustatic sea-level drop caused erosion and a subsequent unconformity (Erwin, 1993). In northern India, the break-up unconformity and subsidence phase is marked by Upper Permian marine deposits transgressing across all underlying structures, including earlier Permian rift basalts (Gaetani and Garzanti, 1991, Garzanti et al., 1998, Kapoor, 1992). The transgression was a gradual one since the sandstone–mudstone facies change occurred earlier towards the outer passive margin to the north. There, thick black shales with phosphatic nodules are late Permian in age (Bagati, 1991, Gaetani et al., 1990, Garzanti et al., 1998), whereas true black shales do not appear until the basal Triassic in Kashmir (see below). The currently accepted definition of the Permian–Triassic boundary, (at the base of the Hindeodus parvus zone), places the main lithological change and the ‘mixed fauna’ of Kashmir in the latest Permian (Yin et al., 2001).

The two sections studied were at Guryul Ravine and near Pahlgam, which are well exposed and easy to collect from. An intermediate section north of Barus is less well exposed in the critical interval (Fig. 1). The general geology of the Kashmir area was first described by Middlemiss (1910) and Wadia (1934) and the Permian–Triassic sections by Middlemiss (1909). More recent works including descriptions of Permian–Triassic boundary deposits in Kashmir are by Ahmad et al. (1978) and Prakash (1978). At Guryul Ravine, Teichert et al. (1960) described the now latest Permian mixed faunas in a 4-m section, and Sweet (1970) determined conodont distributions from relatively widely separated samples (1–2 m apart), without noting lithologies. More recently, Nakazawa et al., 1970, Nakazawa et al., 1975, and Nakazawa and Kapoor (1981) established detailed stratigraphy and faunal changes at Guryul Ravine and adjacent areas (see Kapoor, 1992, Kapoor, 1996, Nakazawa, 1993 for summaries). The Pahlgam area of the Liddar Valley is less well known: its general geology was summarised by Kaul (1976) and Srikantia and Bhargava (1983). Baud et al. (1996) included the sandier sections of the Liddar Valley and the Guryul Ravine section in a carbon and oxygen isotope and sequence stratigraphic study. Nautiyal and Sahni (1976) briefly noted the miospores and phytoplankton from the latest Permian through the Lower Triassic. The Barus section, between Guryul and Pahlgam shows a thicker, sandier Lower Triassic section intermediate in character between the two studied localities (Nakazawa et al., 1975).

No detailed palaeoenvironmental analysis has ever been done on these boundary sections. Nakazawa et al. (1975) made petrographic and sedimentological observations of the entire section, but did not describe the transitional (E1) beds; and we disagree with their interpretation that all the bioclastic beds are deposits of turbidity currents with transported shallow-water fossils. In 1984, we measured the boundary sections at Guryul and Pahlgam, and collected bed-by-bed samples for geochemical analysis in order for collaborators at Berkeley, CA, USA, to test the possibility that a meteorite impact occurred at the boundary. These tests were negative – as has been found in similar boundary sections in China (Clark et al., 1985) and elsewhere in India (summarised by Shukla et al., 2002). In view of this negative evidence, and because we intended returning to Kashmir to complete sedimentological studies on other sections, only a short paper was published on the stratigraphy – in collaboration with V.J. Gupta, whose fossil identifications are now considered highly suspect (Gupta and Brookfield, 1986). In view of these factors we delayed further publication until more work could be done. Unfortunately, the political unrest that has continued in Kashmir since the mid-1980s means that the sections will probably not be visited and studied for some time. In view of this it seems appropriate to now publish our preliminary palaeoenvironmental results, which have some bearing on the interpretation of the faunal changes. Stratigraphic, sedimentological and structural terms and values are standard ones as outlined by Tucker, 1991, Tucker, 1996. Colors cited are those of dry rocks using the Munsell Rock-colour chart of Singewald and Overbeck (1948). Wetting the rock drops its value by about 1, but leaves the chroma unchanged, for example from medium grey (N4) to medium dark grey (N5).