Abstract: The alluvial fan of Yinxian stream is located between Erhai Lake and Diancang Mountain, in the Dali Basin, Yunnan province, which is 2.5km far from ancient Dali city. The surface gradient of the fan ranges from 10°at inner fan, to 6°at middle fan and 2° at outer fan. On the basis of field observation and indoor analysis, the sediment of the fan is classified into four kinds of lithofacies, i.e., intermixed mudrock facies, sandconglomerate facies without mud, cribrose lithofacies and boulder facies. At the same time, the sedimentsize ingredient of each subfacies is analyzed contrastively and the sedimentsize variation law of the whole fan is analyzed in detail, which shows a radial decrease in the grain size from proximal to distal fan with the gradually diminishing velocity. This decreasing law can be fitted well by logarithm curve.

Key words: the fan of Yinxianxi, sedimentary texture, grain size distribution, function matching, Dali Basin, Yunnan