Nature and origin of toreva remnants and volcaniclastics from La Palma, Canary Islands

Abstract

New data pertaining to the origin of volcaniclastic fan delta deposits (the El Time group) and previously unreported toreva remnants (intact landslide blocks) are presented in terms of their relationships with the Quaternary flank destruction of Bejenado volcano on La Palma, Canary Islands. Bejenado volcano is the heavily dismantled relic of a stratocone which developed at the confluence of the Cumbre Nueva embayment and the Caldera de Taburiente collapse amphitheatre. Both of these giant landslide structures formed at around 0.55 Ma after ∼0.2 Ma of gravitational spreading on the western and eastern flanks of the Taburiente–Cumbre Nueva shield volcanic complex. Evidence is presented on the basis of stratigraphic, photogeologic and Geographic Information System studies that both the toreva remnants and the El Time group can be related to the sequential collapses of the north flank and summit region of Bejenado inside the Caldera de Taburiente and the northernmost part of the adjoining Cumbre Nueva embayment. The collapses probably occurred between 0.44 and 0.4 Ma and they involved a volume of around 5 km 3. The toreva remnants display a deformed volcanic architecture, traversed by discontinuities which developed during impact and post-collapse dislocation during slip. They are composed of layer-differentiated scoria with ancillary ankaramite and basanite–tephrite lavas and occasional dikes. They rest upon a thin veneer of sheared volcaniclastic deposits in relative proximity to the disfigured north flank of Bejenado. The collapse event(s) is distinguished by its extremely low mobility due to the deceleration of the debris avalanche mass within the confines of the Caldera de Taburiente. The high friction coefficient ( H/ L=0.22) reflects this topographic factor. Reworking of the avalanche matrix gave rise to the accumulation and offshore progradation of the fan delta deposits at the SW base of Bejenado. Contemporaneous fluvial sediments, sourced by the retrogressive erosion of the pre-Bejenado collapse scarps, were fed into the subaerial confines of the Cumbre Nueva embayment. Piecemeal scarp failure and the extrusion of substratum from beneath the scarps also contributed to the sediment budget. The fluvial sedimentation rates subsequently increased with the onset of Cumbre Vieja volcanism prior to 0.125 Ma. These processes have contributed to the development of the flat-floored submarine channels which extend onto the volcaniclastic apron west of La Palma. These new data give a fresh insight into the role of epiclastic processes, landsliding and volcanism in the morphology and sediment mass balance of an oceanic island volcano system.