Sub-Aqueous Sand Extrusion Dynamics

Ross, Jessica; Peakall, Jeff; Keevil, Gareth M.

A well exposed sand sheet in the Shannon Basin, Ireland provides insights into the dynamics of subaqueous sand extrusion. Sand volcanoes are shown to have erupted contemporaneously following earthquake-induced liquefaction and fluidisation of delta-front mouth bar sands and silts forming an extruded sheet of sand. The sand sheet formed during a single, prolonged eruption event with the interaction of radial gravity currents from the hundreds of vents exerting control on the internal architecture of the extrudite. Sand volcano flank angles are shown to decrease with increasing distance from the vent site unless gravity current interactions equidistant between vents forces premature sediment deposition, artificially increasing flank angles. Discrete sand volcanoes are only a few cm above the sand sheet, and demonstrate that sand extrusions can only form sheets if: (i) multiple vents are extruding coevally, causing gravity currents to interact, or, (ii) topographic forcing, such as channelling, re-directs the otherwise radial gravity currents and results in sheet-like deposition of extruded material away from the vent site. This study provides a new model of sand extrudite formation and shows that detailed sedimentological analysis of sheet-like sands can facilitate the differentiation of primary depositional and extruded sands.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013