The Trollheim alluvial fan and facies model revisited

Abstract

Widely held, fundamental alluvial-fan facies concepts generated in past literature based on the fan, Deep Springs Valley, California, are inconsistent with the actual characteristics of this fan. Previous publications have established, mostly without field study, that the fan consists of debris-flow, sieve lobe, sheetflood, and channel facies; debris-flow and sieve deposits comprise the proximal area, giving way to channel or sheetflood deposits distally. Contrary to these views, clayey and gravelly, matrix-supported, debris-flow deposits dominate all parts of the exposures, suggesting that this fan has been built almost exclusively by mass-wasting events. Channel-fill deposits constitute only a small component of the fan, and most are restricted to the proximal area where they have an inset relationship with debris-flow sequences. The channel facies consists of laterally restricted, clast-supported gravel lenses formed primarily through the winnowing of debris-flow fines by streamflow. The most widespread facies on the surface of the fan is a mantle of varnished, matrix-free pebbles, cobbles, and boulders that commonly are distributed in radially oriented lobes or levees characteristic of debris-flow deposits. This mantle was produced by the surficial winnowing of the fine-fraction from debris-flow deposits primarily by overland water flow. In contrast to previous conclusions, sheetflood and sieve deposits were not identified anywhere on the fan. Additionally, many of the facies depicted in the widely cited Trollheim vertical profile, including ripple-bedded sand, cross-bedded sand, and overbank mud, are not present on, or in stratigraphic cuts of, the fan after which the profile was named.