Radiolarian evidence for the stratigraphy and palaeo-oceanography of the deep-water passive margin of the Indian Plate (Karamba Formation, Indus Suture Zone, Ladakh Himalaya)

Abstract

New radiolarian biostratigraphical information sheds light on the Mesozoic sedimentary, tectonic and palaeoceanographic evolution of a south-Tethyan margin in northern India. Radiolarian cherts studied here form part of deformed deep-water successions within the Indus Suture Zone (Ladakh Himalaya); these are defined as the Karamba Formation and interpreted as the stratigraphically higher and palaeogeographically more distal parts of a passive margin (Lamayuru Group). This formation includes radiolarites, pelagic carbonates, redeposited limestones, ferruginous shales, sandstones, and abundant mafic extrusive rocks.Radiolarian assemblages of early and late Middle Jurassic, mid-Cretaceous (mid-Albian to Cenomanian) and Late Cretaceous (Santonian) age were obtained from four measured sequences. Results indicate: (i) long, intact Jurassic-Cretaceous successions, rather than disrupted mélange, are present in the Karamba Formation; (ii) extensive basic volcanism took place during the Middle Jurassic and can be correlated with an extensional phase affecting the NW Indian continental margin; (iii) non-calcareous radiolarian-rich sediments accumulated during the Middle Jurassic, reflecting the influence either of a broad Jurassic equatorial upwelling zone, or a more localised coastal upwelling system.After drowning of the Zanskar clastic shelf in the late Albian, sedimentation on the distal shelf and upper slope was restricted by bottom current activity. Late Albian-Cenomanian radiolarites of the Karamba Formation are contemporaneous with stratigraphie gaps and glauconite-phosphorite deposition on the Zanskar shelf. A well-developed eastward boundary current off the northern margin of India may have persisted until near the end of the Late Cretaceous. During the late Albian-Cenomanian, current activity and coastal upwelling were probably strong and radiolarian-rich sediments accumulated on the distal margin beneath the CCD. Pelagic limestones accumulated during the Turanian over the entire margin, possibly related to eustatic sea-level rise and consequent reduction in bottom current activity. Intense oceanic circulation apparently was renewed during the Coniacian-Santonian, as indicated by sedimentary hiatuses in the Zanskar shelf succession and accumulation of carbonate-free radiolarian ooze in the Karamba Formation.