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.