Alluvial-Fan Systems

Alluvial fans are conical, lobate, or arcuate accumulations of predominantly coarse-grained clastics extending from a mountain front or escarpment across an adjacent lowland. They represent the coarsest, most poorly sorted, proximal unit in the range of subaerial depositional systems, and commonly merge downdip into finer grained, lower gradient fluvial systems. Some fans, however, terminate directly in lakes or ocean basins as fan deltas, which generally show some degree of distal modification by currents or waves (Fig. 3-1).