THE TRIASSIC OF THE THAKKHOLA (NEPAL) .1. STRATIGRAPHY AND PALEOENVIRONMENT OF A NORTHEAST GONDWANAN RIFTED MARGIN

The Mesozoic sediments of Thakkhola (central Nepal) were deposited on a broad eastern north Gondwanan passive margin at mid-latitudes (28-41 -degrees-S) facing the Southern Tethys ocean to the north. The facies is strikingly similar over a distance of several thousand kilometres f rom Ladakh in the west to Tibet and to the paleogeographically adjacen t north-west Australian margin (Exmouth Plateau, ODP Legs 122/123) and Timor in the east. Late Paleozoic rifting led to the opening of the N eo-Tethys ocean in Early Triassic times. An almost uninterrupted about 2 km thick sequence of syn-rift sediments was deposited on a slowly s ubsiding shelf and slope from Early Triassic to late Valanginian times when break-up between Gondwana (north-west Australia) and Greater Ind ia formed the proto-Indian Ocean. The sedimentation is controlled by ( 1) global events (eustasy; climatic/oceanographic changes due to latit udinal drift; plate reorganization leading to rift-type block-faulting ) and (2) local factors, such as varying fluvio-deltaic sediment input , especially during Permian and late Norian times. Sea level was extre mely low in Permian, high in Carnian and low again during Rhaeto-Liass ic times. Third-order sea-level cycles may have occurred in the Early Triassic and late Norian to Rhaeto-Liassic. During the Permian pure qu artz sand and gravel were deposited as shallowing upward series of sub marine channel or barrier island sands. The high compositional maturit y is typical of a stable craton-type hinterland, uplifted during a maj or rifting episode. During the early Triassic a 20 - 30 m thick conden sed sequence of nodular 'ammonitico rosso'-type marlstone with a 'pela gic' fauna was deposited (Tamba Kurkur Formation). This indicates tect onic subsidence and sediment starvation during the transgression of th e Neo-Tethys ocean. During Carnian times a 400 m thick sequence of fin ing upward, filament-rich wackestone/shale cycles was deposited in a b athyal environment (Mukut Formation). This is overlain by about 300 m of sandy shale and siltstone intercalated with quartz-rich bioclastic grain- to rudstone (Tarap Shale Formation, late Carnian-Norian). The u pper Norian to (?lower) Rhaetian Quartzite Formation consists of (sub) arkosic sandstones and pure quartz arenites, indicating different sedi ment sources. The fluvio-deltaic sandstones are intercalated with silt y shale, coal and bioclastic limestone, as well as mixed siliciclastic - bioclastic rocks. The depositional environment was marginal marine to shallow subtidal. The fluvio-deltaic influence decreased towards th e overlying carbonates of Rhaeto-Liassic (?) age (Jomosom Formation co rrelative with the Kioto Limestone), when the region entered tropical paleolatitudes resulting in platform carbonates.