Abstract

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 V 1 (the axis of maximum concentration), V 3 (the axis of minimum concentration, in effect the pole to the preferred plane of the long axes), and their respective measures of strength, S 1 and S 3 . The S 1 and S 3 values were significantly weaker than those of basal tills, but were similar to those of small-debris flows. The lahar fabrics could be distinguished from those of the latter, however, by the orientation of the V 1 and V 3 eigenvectors. First, the lahar V 1 s showed a statistically significant preference for an upstream dip, whereas the V 1 s of the small flows displayed no dip preference. Second, the mean absolute difference between the V 1 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 V 1 dip of the lahar fabrics was significantly greater than that of the small-debris flows. Fourth, the mean V 3 dip of the lahar fabrics was significantly lower than that of the small 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 V 1 and V 3 orientations. The results suggest that clast fabric may be a more useful property for characterizing debris-flow sedimentology than has been previously assumed.--Modified journal abstract.

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