| Abstract: | Recent stratigraphic studies in southern Tibet provide new information about the timing of the initial collision between the India and Eurasia continental blocks. The stratigraphic and paleontological evidence document dramatic changes in sedimentary facies and microfauna content across the Cretaceous-Paleogene (K/Pg) boundary. In the Zhongba and Gamba areas in southern Tibet, the K/Pg boundary is marked by a major disconformity, separating platform carbonates from overlying terrigenous conglomerates and sandstones. The stratigraphy of the boundary sequences has recently been improved with the recognition of three foraminiferal assemblages. They are: Maastrichtian Orbitoides-Omphalocyclus , Danian Rotalia-Smoutina-Lockhartia and Thanetian Miscellanea-Daviesina microfaunal assemblages. The K/Pg boundary at the Gamba area is placed between the Orbitoides-Omphalocyclus and Rotalia-Smoutina-Lockhartia faunas. In Tingri, Cretaceous Globotruncana and tertiary Globigerina-Globorotalia microfauna demark the position of the K/Pg boundary. The occurrence of terrigenous sandstones and boulder-size conglomerates in the early Paleocene is compelling evidence for tectonic uplift and emergence of the southern margin of the Lhasa block and of the northern margin of the Indian plate. Therefore, supported by biostratigraphic evidence, we argue that the uplift is caused by the onset of continental collision during the earliest Danian. Progressing plate collision resulted in deformation and fragmentation of the Paleocene carbonate platform and deposition of limestone breccias, which we consider as further evidence for tectonic compression as a result of early continental collision during the Thanetian, earlier than indicated by previous studies in the Himalayas. It is the change in the sedimentary facies and depositional environment that provides the earliest evidence and dating of the initiation of the collision process. From studies of sedimentary strata in southern Tibet, the collision of the India and Lhasa continental blocks was initiated at ~K/Pg boundary time (~65Ma). If that is the case, than the major lithofacies changes at the K/Pg boundary observed in the western Tethys, mostly referred to as intrinsic to the eustatic sea level change, has been driven by continental convergence and collision of the Indian and Eurasian plates. |
