A photo-statistical method has been developed to quantify the strength of directional clast fabric in various types of volcaniclastic mass-flow deposits. Fabric strength (R) is defined as the resultant vector length of clast alignment computed from clast angles visible on a vertical outcrop face. R can be obtained from photographs of an exposure via image analysis and statistical assessment with regard to clast number and direction of exposure face vs. paleoflow. Relative to traditional, manual field measurements, the photo-statistical method gives reproducible quantitative results that are independent of subjective clast selection. On average, one hundred and fifty clasts are the minimum number necessary to obtain reliable R values. For determining the strength of directional fabric, it is more accurate to measure randomly collected clasts than only the most elongated ones (normally the practice during manual measurements), because more elongated clasts display a stronger fabric. Clast size generally does not influence clast alignment. R values obtained from lower or upper portions of vertical exposure faces may show significant differences if pronounced imbrication is developed in the lower part of the beds passing upward to purely bed-parallel clast alignment. Bimodality caused by the coexistence of imbricated and bed-parallel clasts tends to reduce R values.
The studied examples include (1) near-vent breccias, (2) block-and-ash flow deposits, and (3) mostly cohesive volcaniclastic debris-flow deposits from 49 selected exposures (24 in Hungary, 6 in France, 4 in Japan, 3 in New Zealand, 2 in Indonesia, and 1 in the U.S.A., Argentina, Chile, Turkey, and Romania, each). Near-vent breccias show a relatively weak fabric (R ≈ 28% on average). Volcaniclastic mass-flow deposits, regardless of their primary or secondary origin, develop a stronger fabric (R ≈ 46% on average) between 0.5 and 1 km from source. At distances more than 1 km, systematic improvement in clast alignment has not been identified. The presented photo-statistical method is applicable to any deposit that displays a preferred fabric.