Late Eocene stepwise seawater retreat from the Pamir-Tian Shan convergence zone (Alay Valley) in the western Tarim Basin, China

Abstract

Inner Asia underwent dramatic changes in sea-land distributions and paleoenvironment in the Cenozoic that were marked by the westward retreat and finally demise of the proto-Paratethys and the subsequent formation of the largest mid-latitude dryland in Central Asia in the Northern Hemisphere. The proto-Paratethys has now retreated to the present-day Mediterranean, but this huge epicontinental sea once extended eastward to the remote Tarim Basin in Central Asia. Although the Tarim and Tajik Basins are today separated by the Pamir salient, they were once the same basin occupied by sea water that belonged to the easternmost part of the Turan Sea in the early Cenozoic. The present Alay Valley that is situated between the Pamirs and Tian Shan was formerly the seawater channel that connected the Tarim and Tajik Basins; since the late Eocene the Valley has experienced a major change in altitude from sea level to 3500 m. The timing and detailed process of the final seawater retreat in the Alay Valley have considerable importance for understanding the interplay between tectonics, surface process, and climate. However, there is still much controversy about the timing of the final seawater retreat from the Tarim and Tajik Basins. In this paper we present a multidisciplinary study of Upper Paleogene strata in the easternmost Alay Valley. Our new magnetostratigraphy, together with the biostratigraphy and the U-Pb age of detrital zircons, indicates that the Upper Paleogene strata have an age range of 40 to 28 Ma. A shallow open sea ended at 40 Ma in both the Tarim and Tajik Basins just after the termination of the Middle Eocene Climatic Optimum. The change from a shallow open sea to an alternative deposition of restricted marine and continental facies began at 40 Ma. Nine marine transgression/regression cycles were recorded in the Alay Valley as indicated by the alternations between restricted marine environment (lagoon) and terrestrial deposition from 39.1 to 37.8 Ma. During this period, the Alay Valley was intermittently occupied by seawater. The final seawater retreat from the Alay Valley was at 37.8 Ma. There might be a diachronous final seawater retreat from the restricted marine environment in the Alay Valley and the Tajik Basin, it was mostly related to the sedimentary hiatuses and/or to the differential uplift and basin filling processes driven by the outward growth of the Pamirs.