REOCCURING MARINE INFLUXES THROUGH LATE PERMIAN TO EARLY TRIASSIC AND THE TRANSITION TO LACUSTRINE CONDITION OF THE SOUTH MARGIN OF THE ORDOS BASIN

The Ordos Basin developed on the southwest margin of the North China block in response to a series of tectonic events. As the North China block and the north Qinling micro block collided and closed the paleo-Qinling Ocean, the Ordos Basin experienced a major transformation from epicontinental sea to a lacustrine basin. The specific timing of this transformation is traditionally considered to be early Permian but is still highly debated. This study intends to constrain the timing of this transformation by analyzing the paleo-environment, paleontology, sedimentology and geochemistry of the Changhsingian (Late Permian) Shiqianfeng Formation to Induan (Early Triassic) Liujiagou Formation at the southern margin of the Ordos Basin. Preliminary results show that (1) the palaeosalinity through the studied interval ranges from 1.271‰ - 4.093‰, suggesting brackish water condition.(2) Geochemical proxies ω(Sr)/ω(Ba) and ω(B)/ω(Ga) range from 0.22 - 0.78 and 0.29 - 2.14, respectively, indicating both marine and terrestrial influences. (3) The detrital zircon U-Pb age distribution mainly has three peak age groups (1864 Ma, 2500 Ma and 441 Ma), corresponding to main sources from the north (North China block) and minor sources from south (North Qinling micro block) of the study area. (4) Carbonate facies developed throughout the studied interval with marine fossils such as Aviculopecten. These results suggest that the paleo Qinling Ocean between the southern margin of the Ordos Basin and the northern Qinling micro Block was not completely closed at least by the end of Liujiagou Period (Early Triassic). During this period, a series of deltaic systems developed in lagoon or offshore environments, resulting in the widespread deposition of both marine and terrestrial facies in the study area. By the end of the Early Triassic to the Middle Triassic, the marine influx gradually waned, and the basin entered the continental phase completely.