NEWLY DISCOVERED TRAVERTINE TERRACES FROM SOUTH-CENTRAL TIBET

A travertine platform measuring roughly 1 km by 0.5 km exists was discovered along a river valley approximately 30 km north of Nyalam in southern Tibet. No active discharge was detected, 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. The geomorphology of the valley is complex and records interaction between prograding alluvial fans 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 composed of gravel and travertine with well-developed rhizoliths rest on the terrace surface. The 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. Many of the terraces terminate in meter-scale “spillovers” up to 10 m tall that are interpreted as travertine draped over the edge of fluvial gravel terraces that developed during different stages of river incision. High Sr87/Sr86 ratios in the travertine (mean of 0.7168) indicate subsurface fluid interaction with radiogenic crystalline rocks of the Greater Himalaya along the trace of the South Tibetan Detatchment System. This fluid/rock interaction suggests that the travertine-depositing spring water either came from depth, passing through Greater Himalayan rocks as it traveled to the surface, or meteoric water traveled to depth, interacted with the crystalline rocks, and then returning to the surface.