Stratigraphy and sedimentary history of the Nepal Tethys Himalaya passive margin

The sedimentary history of the Nepal Tethys Himalaya began with deposition of thick carbonates in the Cambro?-Ordovician, followed by a mixed silicicl astic-carbonate epicontinental succession recording two major deepening eve nts in the Early Silurian and Late Devonian. Fossiliferous carbonate ramp d eposits in the Tournaisian were disconformably followed by while quartzose sandstones and black mudrocks with locally intercalated diamictites derived from sedimentary rocks and deposited in asymmetric tectonic basins ("rift stage"). Break-up in the mid-Early Permian, locally associated with effusio n of tholeiitic lava flows, was followed by a transgressive sandy to shaly, locally coal-bearing or bioclastic unit capped by condensed pelagic carbon ates in the Middle to Late Permian ("juvenile ocean stage"). Subsidence of the cooling stretched crust led close to bathyal water depths in the Olenek ian, but then slowed down in the Middle Triassic to increase again sharply in the Late Triassic owing to renewed extensional tectonic activity and sed iment loading during up- and out-building of the Indian continental terrace . Deposition of tropical platform carbonates finally became widespread in t he middle Liassic ("mature passive margin stage"). The initial fragmentatio n of Gondwana in the Middle Jurassic led to rejuvenation of the Indian crat on and deposition of quartzo-feldspathic hybrid arenites, capped by condens ed oolitic ironstones deposited at warm subtropical latitudes in the late B athonian/middle Callovian. Next, a discontinuous pelagic grey marry limesto ne unit was followed by the ammonoid-rich offshore Spiti Shale in the Late Jurassic. The final disintegration of Gondwana began in the Berriasian, whe n quartzose siliciclastics derived again from the rejuvenated Indian craton and partly from recycling of older elastic successions were followed by th ick deltaic to shelf volcaniclastics documenting eruption of alkali basalts in the Valanginian? followed in the Hauterivian to Albian by more felsic d ifferentiates such as the trachyandesites exposed in the Lesser Himalaya 12 0 km to the south. A widespread drowning episode, fostered by waning volcan iclastic supply during a global eustatic rise, is documented by a major gla uconitic horizon deposited at middle southern latitudes in the late Albian, overlain by "Scaglia-like" pelagic limestones in the latest Albian. The fi nal Dart of sedimentary history, during the rapid northward flight of India and its collision with Eurasia, is not documented anywhere in Nepal due to later erosion of Upper Cretaceous to Lower Tertiary strata. (C) 1999 Elsev ier Science Ltd. All rights reserved.