Abstract

A method for mapping of grain fabric is proposed for analysis of the cryptic internal structure of massive sedimentary units. The method is applied to the analysis of an experimental debris-flow deposit, revealing a number of characteristic features of this type of mass sedimentation. The debris flow was simulated in the laboratory using a channel inclined 30° opening onto a 10° slope, and transverse thin sections were prepared from four longitudinal points in the depositional lobe. Back-scattered electron images of the sections obtained by scanning electron microscopy were processed and analyzed by mapping of grain fabric using an automated image-analysis procedure. Although the samples appear structureless by macroscopic observation, the grain-fabric map reveals a range of sedimentary features, including distinctive lineations from lower-upcurrent to upper-downcurrent in the most proximal section representing synsedimentary thrusts, a steepening-upward trend of grain imbrication angle in intermediate samples with very low-angle imbrication in the basal horizon, indicative of high-shear-rate flow, and complex imbrication features in the most distal samples. This analysis reveals that massive debris-flow deposits actually contain a range of distinctive structures which are characteristic of the mode of deposition and which are not identifiable by visible inspection or analysis of grain size or color. The proposed method is therefore of great utility for the investigation and characterization of massive deposits.