SIEVE DEPOSITION ON STEEP ALLUVIAL FANS

Alluvial fan sedimentology has historically included a type of localized, water-laid deposit referred to as a “sieve deposit” (Hooke, 1967). The concept of sieve deposition was questioned by Blair and McPherson (1992) who suggested that deposits described by Hooke as “possible” sieve deposits on Trollheim fan were actually debris flows. New measurements of clast orientations and deposit stratigraphy were made on Trollheim and nine other fans in the Deep Springs, Eureka, Owens, and Death Valleys of Eastern California. These results clearly differentiate sieve deposits from debris flows. An alignment ratio is here defined as the ratio of the number of elongated clasts with their long axis parallel to flow direction to the number perpendicular. In seventeen deposits, morphologically identified as sieve deposits, this ratio is 0.63 + 0.13 whereas in ten deposits, morphologically identified as debris flows, including debris flow levees and their leading edge noses, the ratio is 1.69 + 0.27. The difference is significant, with a p-value less than 0.05. A longitudinal cross section of a morphologically identified sieve deposit excavated on Trollheim fan exposes a poorly sorted gravel with an average alignment ratio of 0.64, imbricated in places. The truncated half-cone shape, characteristic of sieve deposits, overlies a uniformly sloping debris flow. The trailing face of this deposit has no matrix, typical of sieve deposits. The uphill part does have matrix filling the pore spaces near the surface, but the matrix decreases downward and open pore spaces appear at depth, suggesting infiltration of the matrix. In contrast, an excavated debris flow levee exhibits a profile expected for a surficially winnowed deposit with matrix increasing at depth. A reconnaissance survey of ten fans finds that sieve deposits are more common and larger, with larger clasts, on fans with slopes higher than about 7-10 degrees, and are also more common nearer fan apexes.

Hooke, R. L., 1967, Processes on arid-region alluvial fans: J. Geology, v. 75, p. 438-460.

Blair, T. C., and McPherson, J. G., 1992, The Trollheim alluvial fan and facies model revisited: GSA Bulletin, v. 104, p. 762-769.