Sedimentary evolution and drowning of a passive margin shelf (Giumal Group; Zanskar Tethys Himalaya, India): palaeoenvironmental changes during final break-up of Gondwanaland

Abstract The Lower Cretaceous terrigenous shelf succession of the Zanskar Range (uppermost Spiti Shale and Giumal Group) testifies to major tectono-eustatic events which affected the Tethys Himalaya passive margin during final fragmentation of Gondwanaland. The sudden supply of sand-sized terrigenous detritus in the Lower Takh Formation, deposited in coastal to inner shelf environments probably around late Neocomian to Barremian times, documents a first stage of tectonic uplift and rejuvenation of the Indian continental block. Next, the overall deepening-upward trend recorded by the inner to middle shelf sediments of the largely Aptian Upper Takh Formation indicates relative sea-level rise, culminated with flooding, starvation and deposition of the glauconitic Labar La arenite. This tectono-eustatic event occurred probably around the Aptian-Albian boundary, and was followed abruptly by influx of abundant ‘trachytic’ volcanic detritus, recorded at the base of the Pingdon La Formation. Eruption of alkalic lavas derived from deep-seated mantle sources was confined within the Albian, when volcanic arenites continued to be deposited in inner to middle shelf settings. The local occurrence of up to very coarse-grained, coastal quartz arenites points to episodes of differential uplift and renewed erosion of the Indian Craton. Suddenly, the Zanskar clastic shelf was drowned. Widespread deposition of the Nerak glauco-phosphorite on the sediment-starved outer shelf in the early Late Albian was followed by very discontinuous sedimentation and major stratigraphic gaps at latest Albian to Cenomanian times. Largely palimpsest and condensed arenites (Oma Chu glaucophosphorite) were periodically reworked on the outermost shelf by intruding oceanic currents, which prevented deposition of micrite. Only some decimetres of pelagic oozes (base of the Fatu La Formation) could thus accumulate on the uppermost slope until the Early Turonian, when all across the Zanskar margin sufficient depth was reached to allow rapid sedimentation of foraminiferal mudstones on the upper slope (Chikkim and Fatu La Formations). A complex interplay of tectonic, eustatic and oceanographic processes concurred in the development of ‘drowning unconformities’ at the top of the Giumal clastic shelf. Rapid deepening and waning of both volcanic and quartzo-feldspathic terrigenous detritus are mainly ascribed to the global mid-Cretaceous sea-level rise and to rapid thermo-tectonic subsidence at the end of the short-lived Albian magmatic event, possibly related to a mantle plume rising beneath northern India. Intensification of the east-bound oceanic current off the north margin of India after the final break-up of Gondwanaland was responsible for continuous resuspension and minimal accumulation rates around the shelf-break. The associated coastal upwelling favoured impingement of the oxygen-minimum zone on the outer shelf, with glauco-phosphorite deposition coinciding in time with peak global transgressions and ‘anoxic events’ in the world oceans.