Role of basin‐wide landslides in the formation of extensive alluvial gemstone deposits in Sri Lanka

The tectonically stable central highlands of Sri Lanka and its alluvial valleys are the source areas and sinks, respectively, for one of the most prolific Quaternary gemstone provinces in the world. However, the known ¹⁰Be/²⁶Al cosmogenic‐nuclide‐determined low natural (preanthropogenic) denudation rates of 2–11 mm kyr^?1, and resulting sediment fluxes, are grossly inadequate to deliver the vast throughputs of overburden required to concentrate the known gemstone deposits. Basin‐wide, unstable, slow‐moving channelized landslides and debris flows, aided by biotic factors, are the dominant mechanisms of mass‐wasting on hill‐slopes and bulk delivery of sediment to the alluvial valleys and fluvial networks. Channelization ensures modulated sediment transfer and run‐out during an erosional–depositional continuum. In a selected inventory of landslides, mobilized sediment volumes ranged from less than 1000 cubic metres to a maximum of ～800 000 cubic metres per event. Monsoonal rainfall (both cumulative seasonal and total daily thresholds) is the primary external factor, which interacts with colluvium thickness and steep slopes in triggering landslides. There are three to five ‘threshold’ rainfall events per year in the highlands that can be expected to generate landslides. They can occur under conditions of decreasing daily rainfall as the seasonal total rainfall increases. GIS databases show a very significant spatial overlap and direct causal linkage between several hundred landslide occurrences and the innumerable gem pits and mines in the catchments of the best known mining region of Sri Lanka. Landslide‐associated mass movements, besides providing significant numbers of gemstones to the alluvial valleys over time, are also a fundamental factor in the geomorphic evolution of the rugged central highland landscape. Rainfall‐driven landslide activity may be a natural geological response affecting erosional equilibrium in high‐relief tectonically stable terrains. Climatically forced base level changes will, over time, control sediment storage, removal or reworking in the valleys. Copyright © 2007 John Wiley & Sons, Ltd.