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)) exp erimental debris-flow deposits reveals that strongly aligned, imbricat e fabric can develop rapidly over short distances in mass flows. Pebbl e long axes aligned subparallel to deposit margins as well as subparal lel to margins of surge waves arrested within the deposits. Pebble ali gnment exhibited modes both parallel to (a(p)), and normal to (a(t)), the primary flow direction; intermediate axes dipped preferentially in ward from surge-wave margins (b(i) orientation). Repetitive developmen t of margin-parallel, imbricate fabric distributed across deposit surf aces provides compelling evidence that deposits formed dominantly thro ugh progressive incremental accretion rather than through simple en ma sse emplacement. Pronounced fabric along deposit and arrested surge-wa ve margins reflects significant grain interaction along flow margins. This sedimentological evidence for significant marginal grain interact ion complements theoretical analyses (Iverson, 1997) and other experim ental data (Major, 1996; Iverson, 1997) that indicate that resistance along flow margins is an important factor affecting debris-flow deposi tion. 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 part icle orientation are needed to discriminate mass flow from fluvial gra vel deposits and to unravel depositional history. (C) 1998 Elsevier Sc ience B.V. All rights reserved.