Marine transgressive–regressive cycles and evolution on ancient salt lake in the northwestern Tarim Basin in the Paleocene, Xinjiang Province, China

Multiple large-scale marine transgressive–regressive cycles occurred in the western Tarim Basin during the Late Cretaceous-Paleogene. During these cycles, evaporites deposits were formed repeatedly, of which the most famous is the Paleocene Aertashen Formation. The evaporitic sequences developed during transgressions-regressions are characterized by thick to massive gypsum and anhydrite layers interbedded with mudstone and limestone or argillaceous limestone. The evaporitic unit with the greatest lateral extent must have formed during multiple marine transgressive–regressive cycles rather than a single event. More researchers focus on the marine sedimentary environment, and lithofacies paleogeography of the Basin in that period. The stages and extents of marine transgressions and regressions, which controlled the spatiotemporal changes of ancient salt lake, are unknown at present for the Paleocene succession. In this study, 24 evaporitic profiles (21 boreholes and 3 field outcrops) were studied to identify evaporites sedimentary cycles and obtain data on evaporites and limestone thicknesses. Using these data, 5 small-scale marine transgressive–regressive cycles were identified in the Paleocene strata. The extent of marine transgression expanded gradually from the West Kunlun and South Tianshan piedmonts to the whole basin; in addition, the depositional limits of the ancient salt lake expanded gradually during the first four regressions, but during the fifth regression the lake was present only the Markit slope belt. The depositional centers within the basin migrated from northwest to southeast, and finally reached the northwestern Markit slope belt. For a marine-influenced evaporitic basin, the stages of marine transgression–regression, the extent of each stage, and the spatiotemporal evolution of the extent and depocenter of the ancient salt lake can be reconstructed by identifying evaporites-limestone sedimentary cycles in different areas of the basin.