Characteristics and Geological Significance of Middle and Late Triassic Bivalve Fossils in Ali Zada County,Tibet

Zada County is located in the Zada Basin in the southwest part of the Tibetan Plateau, which is located between the Himalayan Mountains and the Ayira Mountains. The Zada Basin is generally distributed in a NORtheast-to-southwest direction, which is consistent with the northeast-to-southeast tectonic line of the western Himalayan region. It belongs to the tectonic movement product of the collision, subduction and splice convergence of the Indian plate and the Eurasian plate at the end of the Cretaceous period. The structural form. and structure pattern are relatively complex. The basement strata of the basin are mainly Mesozoic Jurassic and Cretaceous sedimentary rocks, including limestone, sandstone, mudstone and shale, and some pre-Mesozoic strata. The overlying strata are mainly Palaeogene-Quaternary fluvial lacustrine sedimentary rocks. The whole middle-late Triassic and even early Jurassic strata are relatively complete, the sedimentary phase transition rhythm is clear, and there are a large number bivalve fossils. The assemblage and distribution characteristics of these bivalve fossils can directly or indirectly reflect the characteristics of environmental changes such as ancient ocean and climate, etc. With the changes of the paleo-ocean and paleo- environment, bivalves are more sensitive to these slight changes in the environment, which are reflected in the biological characteristics of bivalve shell growth patterns and the diversity of combinations (growth grooves or radiating ridges) . Therefore, during the growth process of bivalves, these environmental and climate characteristics will be indirectly recorded in itself, and different environments can correspond to different growth characteristics, that is, different growth patterns reflected on the shell. The growth rate, shell shape and shell pattern of bivalves are more or less closely related to the sea temperature, salinity, climate characteristics, redox conditions, water depth, light and other factors at that time. According to the stratigraphic sequence characteristics from the Mid-Late Triassic to the Early Jurassic in Zada County, we find that this stage includes the complete process from the end of the Middle Triassic transgression to the late Triassic regressive to another transgression in the Early Jurassic, which corresponds to the information of the paleo-marine environment reflected by the characteristics of bivalve fossils