Late Eocene sea retreat from the Tarim Basin (west China) and concomitant Asian paleoenvironmental change

摘要

The Paleogene sediments of the southwest Tarim Basin along the West Kunlun Shan in western China include the remnants of the easternmost extent of a large epicontinental sea. This shallow sea once extended across the Eurasian continent before it retreated westward and eventually separated as the Paratethys Sea. Climate modeling results suggest that this sea retreat is an equally important forcing mechanism as the Tibetan plateau uplift in the aridification of the Asian continental interior and the intensification of the Asian monsoon system. The age and paleogeography of the retreat are poorly constrained, hindering the understanding of its cause and paleoenvironmental impacts. This study reports litho- and biostratigraphic results from two sections recording the last major regression out of the Tarim Basin that is expressed by a regional transition from marine clastics and limestones to continental red-beds. Rich micro- and macrofossil assemblages, including benthic foraminifera, ostracods, bivalves, calcareous nannofossils and organic walled dinoflagellate cysts (dinocysts), indicate a shallow, proximal and marine environment. Strong similarity to assemblages known from Central Asia and Europe confirms that surface-ocean connections extended across Eurasia from the Tarim Basin to the western Tethys during the latest Eocene. Moreover, the recovered fossil associations date the last marine sediments as earliest Priabonian in age (~. 37. Ma; overlap between dinoflagellate Mps Interval Zone and calcareous nannofossil Zone CP 14). The retreat of the sea from the Tarim Basin is time-equivalent with the sea level lowstand at the Bartonian-Priabonian boundary but pre-dates both the Oligocene-Miocene regional uplift of the Pamir mountains and Kunlun Shan and the major eustatic sea-level falls of the Eocene-Oligocene Transition (~. 34. Ma) and mid-Oligocene (~. 30. Ma), which are usually held responsible for the sea retreat. Furthermore, a concomitant and significant aridification step occurs at ~. 36.6. Ma (top of chron C17n.1n) as recorded by regional sedimentary records of the Xining Basin along the northeastern Tibetan Plateau, suggesting that the Tarim Sea served as a significant moisture contributor for the Asian interior.