Seasonal Pattern In the High-Elevation Fluvial Travertine From the Jiuzhaigou National Nature Reserve, Sichuan, Southwestern China

The Jiuzhaigou National Nature Reserve on the Tibetan Plateau (Sichuan, southwestern China) is characterized by the deposition of fluvial travertine in a spectacular array of shoals, waterfalls, pool dams, and multicolored lakes. This is possibly the highest vegetated travertine setting of the world, from 2200 to 2900 m above sea level, an environment sensitive to minimal changes in temperature and precipitation regime. The evolution of the system is driven by two seasonal monsoon climate patterns with a wet spring-summer travertine deposition and then a dry fall-winter characterized by no precipitation or erosion. Spring and phreatic-vadose deposits transition from laminated columnar calcite to clotted micrite encrustation, possibly correlated with a mid-Holocene peak in precipitation and high lake levels in the Northeastern Tibetan area connected to glacial advance. The most peculiar features are the fluvial shoals, a rather uncommon travertine surface consisting of two main superimposed facies of alternating weak- and strong-turbulence water flow. The low-turbulence facies consists of clotted micrite encrustation of mosses and cyanobacteria filaments, and platy calcite crystals covering algal filaments. The high-water-turbulence facies show seasonal alternation of diatom-rich bundles of Phormidium sp. (late spring) with algal Oocardium stratum levels (summerfall). Upslope and downslope of the shoals, the precipitation of calcium carbonate results in the formation of prograding waterfalls and dam-pool systems that encrust macrophytes with microspar and clotted micrite. Encrusted chironomids larvae tubes are present in the waterfall walls up to an elevation of 2860 m, possibly the highest ever recorded. Early diagenetic processes such as dissolution of diatom frustules take place over the span of several years, a slower phenomenon compared to other travertine occurrences. In this high-elevation extreme setting, travertine sand and gravel bar deposits are produced by the exposure to severe weathering of a section of the valley that is bypassed through an underground karst system during the winter dry season. High-elevation travertine probably has a low preservation potential, but it appears to be more sensitive to climate changes than other depositional settings, especially in the Himalayan-Tibetan area, where the extent of the Quaternary glacial advances and retreats is still a matter of debate.