ORIGIN AND SIGNIFICANCE OF TRAVERTINE TERRACES FROM SOUTH-CENTRAL TIBET

A newly discovered travertine platform measuring roughly 1 km by 0.5 km exists along a river valley approximately 30 km north of Nyalam in southern Tibet. This platform did not have an active discharge at the time of study, but there are indications of fairly recent flow in the form of rhizoliths at river level and the entrapment of alluvial fan clasts in travertine at the top of the platform. A complex geomorphological landscape records interaction between growing alluvial fan systems on both sides of the river, the travertine terrace on one side, and a rapidly down-cutting river with associated fluvial terraces. River incision was contemporaneous with travertine deposition, as indicated by a perched gravel deposit that is overlain and underlain by travertine and located roughly 18 m above river level. The perched gravel terrace is overlain by ~1 m of travertine, the top of which defines a terrace surface. Large blocks of travertine rest on this surface and contain a thin layer of gravel as well as well-developed rhizoliths. These blocks were created by upstream erosion of the underlying travertine and the uppermost part of the gravel bed. The rhizoliths in the block are identical to those present along the edge of the modern river where marsh plants with rigid stems of similar shape and size are growing on vegetated bars. This suggests replication of a river-edge ecosystem in time and space during river incision and progradation of the travertine platform. A wide variety of travertine depositional textures are recorded in the platform. These include shrub-like, globular, sponge-like, crystalline-fibrous, and massive forms. On a larger scale, the travertine forms wide, very gently sloping terraces that exhibit pool-and-ridge topography. Many of the terraces terminate in meter-scale “rollovers” that consist of cascading asymmetric folds with long sub-vertical limbs. The widest point of the platform terminates at the modern river with a 10-m-tall cliff of travertine with spectacular rollover folds. This cliff of travertine prograded over the face of the youngest river terrace. The terrace is of similar height and extends along the river beyond the platform to the northwest. The rollovers are therefore interpreted as travertine draped over the edge of fluvial gravel terraces that developed during different stages of river incision.