A debris-flow deposit with multiple plug-flow channels and associated side accretion deposits



A grain-dominant debris-flow deposit of juvenile, basaltic volcaniclastic detritus from a late Eocene-early Oligocene, marine surtseyan volcanic pile in north Otago, New Zealand, contains multiple, semi-circular to U-shaped channel-like structures, or relics thereof, associated with wedges of asymptotic layers of fine and coarse debris. The U-shaped structures are interpreted as channel-like plug flow zones and the associated wedges are interpreted as abandoned parts of the marginal laminar shear flow zone (fine layers) and adjacent plug zone (coarse layers). These appear to have been abandoned by frictional freezing as the plug flow ‘channels’ migrated laterally in an apparently step-like fashion. The debris flow and its deposit appear to have been tabular in geometry, and the development of multiple transport pathways in debris flows is considered in terms of the geometrical ratio of the width of the flow to the dynamic thickness.