Geochemical evaluation of Fenghuoshan Group lacustrine carbonates, North-Central Tibet: Implications for the paleoaltimetry of the Eocene Tibetan Plateau

A sedimentologic, petrologic, and geochemical evaluation of lacustrine carbonates from the Eocene-Oligocene Fenghuoshan Group of north-central Tibet provides insight into the paleoenvironmental and paleolimnological setting of the Hoh Xil basin during the mid-Tertiary. Fenghuoshan lacustrine rocks consist primarily of carbonate mudstones and wackestones. These carbonates are generally less than 3 m in thickness and are intercalated with siliciclastic lacustrine and fluvial/alluvial plain deposits. Individual limestone beds coarsen and contain increasing amounts of siliciclastic material upward. Fenghuoshan carbonates also contain abundant ostracode, bivalve, and charaphyte fossils. Sedimentologic evidence suggests that carbonate deposition occurred in shallow, relatively short-lived lacustrine systems that were subsequently filled by alluvial deposits of coeval fluvial systems. Stable C and O isotopic analysis of Fenghuoshan carbonates show delta(18)O values ranging from -11.7 parts per thousand to -10.3 parts per thousand (Vienna Pee Dee Belemnite [VPDB]) and delta(13)C values between -7.1 parts per thousand and -2.2 parts per thousand (VPDB). The lack of correlation between the two isotopic systems supports the sedimentological interpretation of a shallow, hydrologically open lacustrine setting during the time of carbonate deposition. This interpretation is reinforced by the mineralogy and Mg/Ca molar ratios from Fenghuoshan lacustrine carbonates that indicate fresh water conditions. In addition, petrographic analysis and the delta(18)O composition of samples suggest that Fenghuoshan lacustrine rocks have not undergone significant diagenetic alteration. Collectively, these data indicate that the isotopic composition of Fenghuoshan Group carbonates may be used to infer the oxygen isotopic composition of meteoric water feeding these lacustrine basins at the time of deposition, from which we derive estimates of the paleoaltimetry of north-central Tibet during Middle Eocene time. Model results using the isotopic data from Fenghuoshan carbonates indicate that the hypsometric mean elevation of the drainage basins feeding Hoh Xil lakes was less than or similar to 2 km. Together with estimates of the Eocene paleoaltimetry from the Lunpola basin to the south, these results provide the first direct evidence for the differential uplift of the northern margin of the Tibetan Plateau.