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.