Preliminary Research on the Mesozoic Lithofacies-Palaeogeography in the Chalangla Area, Qiangtang Basin, Northern Tibet

For the first time, the paper systematically rebuilds the Mesozoic lithofacies and palaeogeography in the Chalangla area of Qiangtang Basin in Northern Tibet by using methods of stratigraphically dominant facies/subfacies and integrated analyses. This is on the basis of the measured sections and route geologic survey profiles as well as sedimentary facies analyses. Marine-continental transition facies were dominant in Late Triassic, forming regression sedimentary cycles and consequently developing delta facies, coastal facies, and littoral-neritic facies belt from NE towards SW. In the Middle Jurassic epoch developed neritic facies deposits, consequently delta-platform-neritic continental shelf facies occurred from NE towards SW and displayed in NWW～SEE direction. Twice marine transgression peaks occurred at J 2 q Period and J 2 b Period,and constituted two sedimentary cycles from marine ingression to regression. In the Late Jurassic epoch developed sea-continental transitional facies,which were delta, tidal flat, platform facies belts and displayed in NW～SE direction.In the Cretaceous period discrepancy in lithofacies and palaeogeography was present from the early to late. In the early stage developed platform edge shallow facies to restricted platform facies on the background of remnant sea, which formed the third marine transgression peak. After rapid overall swelling, in the late stage occurred alluvial fan-braided river facies sedimentary cycles made of red polyterrigenous molasse deposits with multi-source supplying. The Mesozoic provenance was mainly the central uplift belt in the north, and secondly the highland in the east and uplift region in the west. In the process of sedimentary evolution, sedimentary source areas were different in different geologic ages, and the terrain in general,was characteristic of being higher in north and east but lower in south and west.