Bypassing and diachronous deposition from density currents: evidence from a giant regressive bedform in the Poris ignimbrite, Tenerife. Richard J. Brown* & Michael J. Branney Geology Department, University of Leicester, University Road, Leicester, LE1 7RH, England *current address: Osservatorio Vesuviano, Via Diocleziano, 328, 80124 Napoli, Italia ABSTRACT Exceptional longitudinal exposure through an ignimbrite on the southern flanks of Las Cañadas volcano, Tenerife reveals a giant regressive bedform. It comprises sourceward-dipping lenses, 50 m long and up to 30 cm thick, of massive and diffuse-bedded lapilli-tuff. Its architecture, and surrounding field relations, indicate that at any one time, deposition was restricted to a c. 50 m long longitudinal depositional zone, bounded both upslope and downslope by extensive bypassing zones. The regressive architecture of the giant bedform shows that this restricted depositional zone gradually migrated sourceward as the current passed. Localised impersistent scours indicate that the current was locally erosive. However, at most locations the current seems to have been neither depositing nor significantly eroding, and most of the pyroclastic load bypassed subaerial slopes entirely, and entered the Atlantic ocean. Any single vertical section through the internally diachronous ignimbrite is representative of only a short phase of the current’s duration. However, the diachroneity would not be apparent at smaller (c. 10 m long) exposures because of the extremely low-angle, diffuse nature of the layering. Similarly protracted and complex flow histories elsewhere may be overlooked at typical sized field exposures of ignimbrites, turbidites and lahar deposits, leading to significantly underestimated flow durations and volumes.