Geochemical Evaluation of Fenghuoshan Group Lacustrine Carbonates, North‐Central Tibet: Implications for the Paleoaltimetry of the Eocene Tibetan Plateau

Abstract

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 d 18 O values ranging from Ϫ11.7‰ to Ϫ10.3‰ (Vienna Pee Dee Belemnite [VPDB]) and d 13 C values between Ϫ7.1‰ and Ϫ2.2‰ (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 d 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 Շ2 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.

Constraints on the early uplift history of the Tibetan Plateau

“…Paleoaltimetry estimates from the Hoh Xil region are equivocal. Cyr et al (18) used oxygen isotope values from lacustrine carbonates from the FG and modeled monsoondominated isotopic lapse rates to argue that the HXB was Ϸ2 km high during the Late Eocene, whereas DeCelles et al (22) reevaluated these data by using lower, empirically based lapse rates from central Tibet and argue that the HXB was 4.7-5.0 km high during the Late Eocene.…”

“…How such high topography, which should have an effect on climate, monsoon intensity, and ocean chemistry (1)(2)(3)(4)(5), has developed through geologic time remains disputed. Various lines of investigation, including evidence from the initiation of rift basins (6), potassium-rich (K-rich) volcanism (7), tectonogeomorphic studies of fluvial systems and drainage basins (8), thermochronologic studies (9), upper-crustal deformation histories (10,11), stratigraphic and magnetostratigraphic studies of sediment accumulation rates (12), paleobotany (13), and oxygen isotope-based paleoaltimetry (14)(15)(16)(17)(18)(19)(20)(21)(22), have suggested different uplift histories. Authors of recent geologic studies (11) have proposed that significant crustal thickening (and by inference, surface uplift) in the Qiangtang terrane occurred in the Early Cretaceous [Ϸ145 mega-annum (Ma) age], followed by major crustal thickening within the Lhasa terrane between Ϸ100 and 50 Ma ago.…”

Asynchronous responses of East Asian and Indian summer monsoons to mountain uplift shown by regional climate modelling experiments

“…They inferred that it is the orographic insulation of low entropy (Wang et al 2008b); 2. Fenghuo Shan (Cyr et al 2005); 3. Lunpola Basin (Rowley and Currie 2006); 4.…”

Climate-driven environmental change in the Zhada basin, southwestern Tibetan Plateau

“…Wang et al, 2008a). In contrast, other lines of evidence indicate that the southern Tibetan Plateau has been at high elevations since at least the Middle Miocene (Garzione et al, 2000a;Rowley et al, 2001;Spicer et al, 2003;Currie et al, 2005;Saylor et al, 2009) and central Tibetan Plateau since at least the Oligocene (Cyr et al, 2005;Graham et al, 2005;Rowley and Currie, 2006;DeCelles et al, 2007;Dupont-Nivet et al, 2008). These paleoelevation studies also show that uplift predated widespread Late Miocene climate change (see Molnar, 2005, for a summary of evidence for Late Miocene climate change).…”

“…However, any such models must take into consideration long-lived high elevations in the southern and central Tibetan Plateau (Garzione et al, 2000a;Rowley et al, 2001;Currie et al, 2005;Cyr et al, 2005;Rowley and Currie, 2006;DeCelles et al, 2007;Dupont-Nivet et al, 2008;Saylor et al, 2009). 2.…”