Abstract

Exceptional longitudinal exposure through an ignimbrite sheet on the southern flanks of Las Cañadas volcano, Tenerife, Canary Islands, reveals a giant regressive bed form, >520 m long. It comprises sourceward-dipping diffuse lenses, ∼50 m long, of massive and diffuse-bedded lapilli tuff. Its architectural relationships indicate that at any one moment in time, deposition was restricted to an ∼50-m-long longitudinal depositional zone, bounded both upslope and downslope by extensive bypassing zones. The regressive architecture shows that the zone of deposition gradually migrated sourceward as the current passed. Nonpersistent scours indicate that the current was locally and momentarily erosive, although at many locations there is little evidence for substantial erosion or deposition, and most of the pyroclastic load seems to have entirely bypassed subaerial slopes to enter the Atlantic Ocean, ∼20 km from the source. The ignimbrite is diachronous, and any single vertical section through it represents only a short phase of the current's protracted depositional history. Similarly protracted and complex flow histories elsewhere may be easily overlooked at typical field exposures of ignimbrites, turbidites, and lahar deposits, leading to significantly underestimated flow durations and volumes.

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