Abstract: Most knowledge about the Cretaceous-Paleogene strata in the Tarim Basin is mainly inferred from the outcrops at the basin’s margin，but first time in the basin. The formation mechanism of the Paleocene basal conglomerate was determined using geochemical isotopes of the breccia from well PBX1 in the southwestern Tarim Basin. The results showed that the global K/Pg (i.e., Cretaceous/Paleogene) clay layer boundary was located in the middle of the Paleocene basal conglomerate at the depth of 7066.75m in well PBX1. In the Late Cretaceous, associated with volcanic activities and earthquakes, the caldera in the PBX1 well block formed an annular depression with large elevation differences in response to the Pamir block collision. As a result, the collapsed breccia with storm tide genesis deposited at the periphery and inside of the depression, characterized by syn-sedimentary deformation. During the Paleocene, multiple sets of interbedded carbonate and collapsed breccia deposited in response to multi-phased transient transgression-regression cycles. The transportation of breccia exhibited near-source accumulation/extremely close or in-situ rapid accumulation. The studied region is located in the eastern end of the Tethys Sea, the Late Cretaceous-Early Paleocene breccia is of great significance for reconstructing the paleogeography of the Tarim basin in Tethys.
Key words: South-western Tarim；Paleogene system；Breccia；Attribution and genesis