14.6 MA ARIDIFICATION AND DEVELOPMENT OF THE ANCESTRAL TAKLIMAKAN DESERT BETWEEN THE PAMIR AND TIAN SHAN OROGENS, WESTERN CHINA

The Taklimakan Desert in western China is the second largest shifting sand desert on Earth, and formed during the Cenozoic between the Tibetan Plateau, the Tian Shan, and the Pamir Mountains. The onset of desert formation has been debated between the Eocene, early Miocene, late Miocene, or Pliocene, with each hypothesis having profound implications for the climatic and tectonic evolution of this region. We describe a 3800 m-thick stratigraphic section in the northwestern Tarim Basin that provides new evidence for development of the ancestral Taklimakan Desert. Magnetostratigaphy defines 48 magnetozones that define the age of these strata between 17.5 and 2 Ma. Noteworthy sedimentologic intervals include 50 m of gypsiferous shale between 14.6-14.2 Ma followed by >1000 m of cross-bedded, eolian sandstone between 12.2-7.2 Ma and the appearance of the Xiyu Formation conglomerate at 2 Ma. Paleocurrent data (n=137) combined with preliminary detrital zircon geochronology suggest a dramatic shift in sediment source from the Pamir to Tian Shan at 14.6 Ma, coincident with the first evaporate deposits. These data suggest that growth of the Tian Shan and collision with the Pamir after 15 Ma may have closed-off moisture into the Tarim Basin, creating a rain-shadow on the western Tarim Basin that dried up lakes and rivers and created a sand-source for eastward migrating dune systems. The appearance of gypsiferous playa sediments at 14.6 Ma coincident with a change in sediment provenance suggests uplift of the Tian Shan is at least partially responsible for the changing climate, although global cooling during the middle Miocene transition at ~14 Ma enhanced aridification and promoted formation of the erg system. By 12.2 Ma, erg development had begun and is ongoing today, although the outward growth of the Tian Shan since ~7 Ma may be pushing the erg depocenter southward over time. These data provide direct evidence of a large-scale, shifting sand desert in western China at 14.6 Ma, and highlight the influence of tectonics on regional climate.