Talus Fabric and Particle Morphology on Lassen Peak, California

ABSTRACT

Particle morphology and fabric were examined on three talus slopes with different aspects (SW, S, SE) at Lassen Peak, California, to analyze the spatial variability of clast characteristics and determine the geomorphic processes causing it. The slopes had concave profiles and a pronounced particle sorting by size and shape. Sorting was not gradual; instead, a basal fringe of clasts larger and more equidimensional than those on the talus above was identifiable with statistical tests. Particle fabrics were characterized by a subhorizontal girdle coincident with the local slope, dominant modes parallel or oblique to the slope, and a high percentage of imbricated clasts. The basal fringe and the fabrics were most pronounced in the SE and S taluses.

The observed sedimentological patterns suggest that rockfall is the dominant agent of clast deposition. However, the fabrics indicate that many particles glide down the talus with their long axis parallel to the slope. This occurs commonly on the upper slopes, mostly covered by fine debris, but cannot take place on the blocky basal talus. A persistent snow cover on the SE and S flanks provides a smooth surface for particles, which are segregated by size and shape while descending. Large and spherical clasts reach the toe slope, forming a basal fringe, while small and flat stones stop on the upper talus. Other processes contributing to particle transport include miniature debris flows, dry debris slides, and needle-ice growth.