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 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-andash 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 ഠ 28% on average). Volcaniclastic mass-flow deposits, regardless of their primary or secondary origin, develop a stronger fabric (R ഠ 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.

Granular materials composed of shape-anisotropic grains

“…Traditionally, such measurements were performed on the basis of manually selected clasts in a vertical cross section of the sediment layers. Karátson et al [135] developed a photo-statistical method to quantify the strength of directional clast fabric, i.e. the vector length of clast alignment on vertical outcrop faces, to determine orientational distributions of various types of volcaniclastic mass-flow deposits.…”

Grain-size distribution of volcaniclastic rocks 1: A new technique based on functional stereology

“…Capaccioni et al, 1997;Karatson et al, 2002). Image analysis can be used from any medium, including field photographs or scans of rock slabs and thin sections (Fig.…”

Clast fabric examination of impact‐generated breccias, borehole LB‐07A, Bosumtwi, Ghana

“…Since the borehole televiewer imagery is referenced to geomagnetic north it is possible to bring the optical photographic images into an absolute reference frame by comparing the images (Hunze and Wonik 2007). Karatson et al (2002) provide an excellent review of the application of photo-imaging to clast fabric orientation estimation. Specific problems identified by Karatson et al (2002) include the effect of clast size, the number of clasts, and the bias introduced by view angle.…”

“…Karatson et al (2002) provide an excellent review of the application of photo-imaging to clast fabric orientation estimation. Specific problems identified by Karatson et al (2002) include the effect of clast size, the number of clasts, and the bias introduced by view angle. Most of the imagery analyzed by Karatson et al (2002) used one view plane ideally chosen to be the plane of maximum fabric.…”

“…Specific problems identified by Karatson et al (2002) include the effect of clast size, the number of clasts, and the bias introduced by view angle. Most of the imagery analyzed by Karatson et al (2002) used one view plane ideally chosen to be the plane of maximum fabric. This is the first study to present a detailed analysis of clast fabric as revealed by digital optical photography of oriented core samples.…”