Magnetostratigraphy and palaeoenvironmental records for a Late Cenozoic sedimentary sequence from Lanzhou, Northeastern margin of the Tibetan Plateau

Geological studies indicate that several Tertiary fluvial–lacustrine basins existed in northwestern China. Progressive uplift of the Tibetan Plateau forced these Tertiary basins to dry, and eventually became accumulation areas for aeolian sediments. Continuous sedimentary sequences consisting of alluvial–fluvial sediments and aeolian Red Clay occurred in the Lanzhou area. We used palaeomagnetic and palaeoenvironmental techniques to investigate a late Tertiary section that has a Quaternary loess cover. Our results indicate that the area was a low altitude basin with a fluvial–lacustrine environment before ~ 7.2 Ma. Starting from ~ 7.2 Ma, the basin became a dry land and was exposed to aeolian accumulation. The presences of unconformities and sedimentary facies changes suggest that strong tectonic activity occurred during the interval of 3.5–1.8 Ma, which forced the Lanzhou Tertiary Basin to break up and became elevated to its present altitude. Gradual variations in sediment colour, grain-size and magnetic susceptibility imply that the Lanzhou area experienced progressive cooling and drying in the Late Cenozoic due to local tectonic uplift and global cooling.