Morphological feature analyses of the Prestige half-graben on the SW Galicia Bank

Abstract

Morphological analysis was carried out on the SW flank of the Galicia Bank, as part of detailed geological studies of the half-graben where the Prestige oil tanker wreckage is located. Broad multibeam echosounder (SIMRAD EM-12S-120) and seismic reflection (Airgun) data were collected on board RV Hespérides. Three groups of morphological features were identified as: tectonic, erosive and depositional. The main tectonic morphological feature is a N–S oriented fault scarp in the eastern part of the study area. Main erosive features are: gullies, slide scars, E–W oriented inter-lobe channels and a NNE–SSW oriented Main Channel. Dominant depositional morphologies comprise: thirteen E–W oriented sedimentary wedges, seven large E–W to NE–SW oriented sedimentary lobes, and the relatively smooth sedimentary layer covering the eastern and western ridges that comprise the Galicia Bank's SW flank. Characterization, classification and distribution of these features indicate that the study area constitutes a half-graben structure comprising a N–S oriented normal fault scarp abruptly connecting steep gradients in the eastern area with the gentle slope of the central area. A thick but narrow wedge of sediment has accumulated at the foot of the scarp as a result of mass-movement processes that govern the formation of a series of short-lived wedges, overlapping lobes and also channels. This particular kind of slope apron system corresponds to a tectonic-controlled drainage pattern from a sharply tilted, hanging wall block, supplied by sediment from the ridge. Finally we can conclude that the Prestige stern and bow are located at the steep fault scarp, close to a channel head, and at the axis of inter-lobe channels developed by mass-movement processes. Thus, morphologic, tectonic and sedimentary factors can be considered as potential hazards.