Sedimentary environments of middle Eocene sediments in the northern Qiangtang Basin, northern Tibetan Plateau: Implications for paleoclimate and paleoelevation

As the largest and highest continental plateau on the Earth, the Tibetan Plateau has attracted worldwide attention to its formation and its remarkable paleoenvironment changes. Here we used the new stable isotope results of the lacustrine carbonate cement and pollen assemblages from the middle Eocene Suonahu Formation that was drilled from the QD17 well to infer the middle Eocene paleoclimate and paleoelevation of the northern Qiangtang basin in the central Tibetan Plateau. According to geological logging of the QD17 well, the sedi-mentary succession in the Suonahu Formation consists of purple-red calcareous mudstone, marlstone, siltstone, white gypsum, and gray-green mudstone. Analyses of the lithofacies indicate that the Suonahu Formation was deposited in lacustrine-evaporation environments. Both petrographic observations and isotopic data suggest that the lacustrine carbonate cement likely experienced an early-stage digenesis. The results of pollen records show that palynological taxa are mainly dominated by Ephedripites-Betulaceoipollenites-Pterisisporites and Ephedripites- Nitrariadites-Taxodiaceaepollenites assemblage, which suggest the vegetation types of the Suonahu Formation during the depositional period were mainly subtropical-temperate. The delta 18Opsw values of paleo-surface water reconstructed from the lowest (i.e. the least evaporated) delta 18Oc values (-10 parts per thousand) of the weakly altered carbonate cement are used to make the minimum estimation of the average paleoelevation of the drainage basin. The paleoelevation of the northern Qiangtang terrane is reconstructed as about 2830 m by the middle Eocene-early Oligocene, approximately 2200 m lower than the present-day elevation.