ABSTRACT

Clast-orientation measurements were made at 11 sites on Mount St. Helens lahar deposits along the North Fork of the Toutle River, Washington. Schmidt nets show that fabrics tend to be unimodal, though relatively weak. A quantitative comparison of the fabrics with those reported for basal tills and smaller debris flows was made using the eigenvalue method, which calculates eigenvectors V₁ (the axis of maximum concentration), V₃ (the axis of minimum concentration, in effect the pole to the preferred plane of the long axes), and their respective measures of strength, S₁ and S₃. The S₁ and S₃ values were significantly weaker than those of basal tills, but were similar to those of small-debris flows. The lahar fabrics could e distinguished from those of the latter, however, by the orientation of the V₁ and V₃ eigenvectors. First, the lahar V₁s showed a statistically significant preference for an upstream dip, whereas the V₁s of the small flows displayed no dip preference. Second, the mean absolute difference between the V₁ azimuth and the local-flow direction was significantly larger than that for the small flows, indicating a greater tendency for transverse orientations in the lahar fabrics, particularly those of clast-supported samples. Third, the mean V₁ dip of the lahar fabrics was significantly greater than that of the small-debris flows. Fourth, the mean V₃ dip of the lahar fabrics was significantly lower than that of the sma l flows, indicating a significantly steeper dip of the preferred plane of the long axes, particularly in the matrix-supported samples. The lahar fabrics could also be distinguished from those of basal tills by means of the V₁ and V₃ orientations. The results suggest that clast fabric may be a more useful property for characterizing debris-flow sedimentology than has been previously assumed.