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Saier Wu, Jian Chen and Zhijiu Cui
Large fan shaped debris-flow deposits occur at the piedmont west of Benzilan, in the upper stream of the Jinsha River, southwest China. The accumulation is composed of alternation of debris-flow units and reddish gravel soil units, seemly showing a binary structure. The debris-flow deposit has a mean thickness of 100 m. We did analysis on particle size, major element, clay mineral, pollen and electronic spin resonance (ESR) dating for samples from the debris-flow accumulation. Our study shows that the reddish gravel soil was in fact the debris flow material and its apparent differences from the debris flow material, especially color, was due to weathering. It was a relative dryhot climate to weather the upper part of the debris flow body into the reddish gravel soil. Evident chemical difference between the soil and debris-flow units was caused essentially by carbonate dissolution from soils. The debris-flow sequences indicate that the climate of the upper Jinsha River valley during the Early Pleistocene was characterized by a remarkable wet-dry alternation and would be warmer than today. The study area would be uplifted by 1300 m since the Early Pleistocene.