Preferred Clast Orientation in Volcaniclastic Mass-Flow Deposits: Application of a New Photo-Statistical Method

Abstract

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 vec- tor length of clast alignment computed from clast angles visible on a vertical outcrop face. R can be obtained from photographs of an ex- posure via image analysis and statistical assessment with regard to clast number and direction of exposure face vs. paleoflow. Relative to tra- ditional, 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 mini- mum number necessary to obtain reliable R values. For determining the strength of directional fabric, it is more accurate to measure ran- domly 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 imbri- cation is developed in the lower part of the beds passing upward to purely bed-parallel clast alignment. Bimodality caused by the coexis- tence 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., Argen- tina, Chile, Turkey, and Romania, each). Near-vent breccias show a relatively weak fabric (R