FORMATION AND EVOLUTION OF THE MESOZOIC AND CENOZOIC DEEP-WATER SEDIMENTARY BASINS ALONG THE YARLUNG ZANGBO RIVER(Ⅱ): DYNAMICS OF THE HIMALAYAN CARBONATE PLATFORMS

The initiation, development and termination of the Himalayan carbonate platforms are closely related to the dynamic evolution of the Tethyan orogenic belt. In the Triassic, the carbonate platforms were firstly born on the shelf margins in the Nyalam area. In the meantime, they were also developed on and near the critical growing surfaces of the carbonate platforms because of the interference of the terrigenous clastics. Later in the Early and Middle Jurassic, the carbonate platforms, influenced by the tectonic subsidence and sea-level changes, evolved cyclically from the low-energy subtidal zone to the high-energy shallowing rimmed platform. A sequence of especial carbonate "shedding flow" deposits were laid down in the slope-basin environment of the platform margins. In the Late Jurassic, the carbonate platforms were covered by black shale and drowned to the death as a result of the rapid thermal subsidence during the early stage of the passive continental margin development. Till the Early Cretaceous, the platforms on the shelf margins may be characterized by the isolated platforms, a lot of which were destroyed and slumped into the marginal deeper basins, and then the platforms became the distally steepened ramps near the landward side of the Indian Continent. Beginning from the Late Cretaceous, the carbonate platforms were well developed mainly in the Gamba area, with deepening-upward sedimentary sequences in the foreland wedges. At the beginning of the Tertiary, the carbonate platforms passed gradually into ramps, belonging to the carbonate platorm sediments far from the orogenic side of the foreland basins. The termination of the Himalayan carbonate platforms may be attributed to the uplifting and exposure by the orogenic movements.