MUDFLOW ALLUVIAL FANS OF THE ATACAMA DESERT IN CHILE

Along a 200 km transect of the Atacama Desert in northern Chile a series of mudflow-dominated alluvial fans derive from the volcanic Andean foothills, flowing onto the hyperarid interior basin at about 1100 m elevation. The fan systems, centered at about 20.5°S, 69.4ºW are active at about decadal frequency. Individual flows extend 20-30 km across the basin, with individual flows averaging about 0.5 meters deep but ~1 km across. Individual deposits are typically 5-25 cm thick with a matrix averaging roughly 15% expandable clay, 40% silt, and 45% sand, mixed with variable amounts of fine gravel. Near the fan apex flows are partly channelized, but downstream the flows occur as shallow, braided flows with extensive overbank deposits. The mudflows also transport a small proportion of coarse gravel to boulders, deposited as bars and one- or two-boulder wide levees. Some of the boulders are rip-up clasts from earlier events. The coarse component is primarily transporting by rolling across the smooth fan surface. The mud deposits are susceptible to wind erosion, so that older flows become covered by a dark armor of granules to pebbles. Individual flows only partially follow the path of earlier flows, with frequent creation and abandonment of individual channels. The thick fan deposits are therefore composed of thousands of thin, interlaced mud deposits with granule or pebble deflation lags and scattered boulders and boulder bars associated with channels.