Pebble orientation on large, experimental debris-flow deposits

Replicable, pronounced orientation of discoid pebbles (greater than or equal to 8 mm) embedded on surfaces of large (similar to 10 m(3)) experimental debris-flow deposits reveals that strongly aligned, imbricate fabric can develop rapidly over short distances in mass flows. Pebble long axes aligned subparallel to deposit margins as well as subparallel to margins of surge waves arrested within the deposits. Pebble alignment exhibited modes both parallel to (a(p)), and normal to (a(t)), the primary flow direction; intermediate axes dipped preferentially inward from surge-wave margins (b(i) orientation). Repetitive development of margin-parallel, imbricate fabric distributed across deposit surfaces provides compelling evidence that deposits formed dominantly through progressive incremental accretion rather than through simple en masse emplacement. Pronounced fabric along deposit and arrested surge-wave margins reflects significant grain interaction along flow margins. This sedimentological evidence for significant marginal grain interaction complements theoretical analyses (Iverson, 1997) and other experimental data (Major, 1996; Iverson, 1997) that indicate that resistance along flow margins is an important factor affecting debris-flow deposition. The fabric on the experimental deposits demonstrates that debris flows can develop strongly imbricate particle orientation that mimics fabric developed during fluvial deposition. Particle shape and local stress fields appear to have more control over fabric development than does general depositional process. Other criteria in addition to particle orientation are needed to discriminate mass flow from fluvial gravel deposits and to unravel depositional history. (C) 1998 Elsevier Science B.V. All rights reserved.