Abstract
Ten undisturbed samples of scarp-derived colluvium from the 1983 Borah Peak, Idaho, fault scarp were subjected to laboratory grain-size and fabric measurements to characterize typical colluvium from a Basin and Range fault scarp. Colluvial deposits consisted of poorly-sorted pebble gravels (mean diameter -0.3 to -3.75 phi ; 1.2 to 13.5 mm) with 25-79% matrix (< 4 mm). Stereograms of long-axis orientations of 115 clasts from each colluvial wedge yielded girdle-type distributions, with typical eigenvalue ratios of In S1/S2 = 0.05-0.50 and In S2/S3 = 0.3-1.0 These girdle distributions indicate preferred orientation in angle-of-repose planes coincident with colluvial wedge depositional planes. Short-axis stereograms display single or dual maxima with mean bearing parallel to transport direction and plunge perpendicular to the colluvial wedge surface. Clast fabric strength is not strongly correlated with scarp aspect (corr. coeff. = 0.03 to 0.68) or with the slope of the faulted geomorphic surface (corr. coeff. = -0.16 to +0.57), but it improves with increasing matrix content (corr. coeff. = -0.04 to -0.42). Fabrics in measured wedges are weaker than those reported for other colluviums and talus and resemble fabrics in debris flows. However, the stronger girdle tendencies and distinct orientation subgroups in scarp-derived colluvium should distinguish it from suspected debris flow deposits in fault-zone exposures.
