The continental shelf of NW Iberia is of particular interest due to the complex North Atlantic rifting tectonics and modern oceanographic processes that led to its current geomorphological configuration and sediment dynamics. The shelf forms a narrow slow-dipping north-bearing geomorphological structure with a well-defined shelf break located at water depths of 160-180 m. It is a continental margin with sedimentation rates of about 1.5 +/- 2.0 mm a(-1) (the highest values in recent bottom sediments of the ria) subjected to a highly energetic seasonal regime of waves and tides, seasonal upwelling and coast-parallel currents, significantly modulated by the North Atlantic Oscillation. Sediment provenance is dominated by inputs of continental sediments via runoff and riverine discharges and, to a lesser extent, by an upwelling-enhanced bioclastic input. Present-day sedimentation in the area occurs closely associated with two geomorphologically distinct sedimentary environments: the rias inner shelf sedimentary systems and the open continental shelf, and their subsequent evolution since the last glacial.

Abstract The continental shelf of NW Iberia is of particular interest due to the complex North Atlantic rifting tectonics and modern oceanographic processes that led to its current geomorphological configuration and sediment dynamics. The shelf forms a narrow slow-dipping north-bearing geomorphological structure with a well-defined shelf break located at water depths of 160–180 m. It is a continental margin with sedimentation rates of about 1.5±2.0 mm a−1 (the highest values in recent bottom sediments of the ría) subjected to a highly energetic seasonal regime of waves and tides, seasonal upwelling and coast-parallel currents, significantly modulated by the North Atlantic Oscillation. Sediment provenance is dominated by inputs of continental sediments via runoff and riverine discharges and, to a lesser extent, by an upwelling-enhanced bioclastic input. Present-day sedimentation in the area occurs closely associated with two geomorphologically distinct sedimentary environments: the rías inner shelf sedimentary systems and the open continental shelf, and their subsequent evolution since the last glacial.

An echo-character analysis of the oil tanker Prestige wreck area was undertaken using high-resolution marine geophysical methods (TOPAS and airgun seismic-reflection profiles, multibeam echosounder and TOBI sidescan sonar). Integration and comparison of the results using all these methods is presented given some practical applications for indirect near-surface and seafloor interpretations. Ten different echo types were identified and grouped into four main classes: I) distinct; II) indistinct; III) irregular and IV) hyperbolic. Echo-character distribution enabled us to determine recent sedimentary processes in the area. Two major depositional systems can be found through the identification of these sedimentary processes: a) a slope depositional system (SDS) located in the eastern and central area, where mass-movement processes are dominant and b) a turbiditic Main Channel depositional system (TDS) located in the western area where channel-related processes are dominant. Both of these interact over the half-graben structure of the southwestern sector of the Galicia Bank, where the Prestige wreck is located. Within the SDS, erosive and depositional mass-movement processes characterised a complex depositional system. Erosive processes occur on the fault scarp, channels head, inter-lobe channels and distal part of the sedimentary lobes. Moreover, depositional processes take place on the top of the fault scarp, sedimentary wedges, sedimentary lobes, and on the west flank of the Main Channel. Both depositional systems interact, but the SDS should be more active during fault-scarp reactivation periods, through relief rejuvenation and new exposed deposits. Microearthquake activity would favour the available materials, being weathered, eroded and transported by mass-movements. In such a situation, the TDS acts as the main collector of eroded sediment derived from the fault scarp throughout the SDS being responsible for its evacuation into the Iberian Abyssal Plain. However, outside of fault reactivation periods, the SDS is less active (such as during the present situation).

Potential geological hazard assessment has been carried out in the area where the Prestige vessel was sunk using a broad database that comprises: multibeam, high and ultra-high resolution seismic profiles, gravity cores, onland seismicity stations and Ocean Bottom Seismometers (OBS). The main results of this study indicate that among the geologic factors that can be considered as potential hazards, four main categories can be differentiated based on their origin: morphologic, sedimentary, tectonic, and seismicity. Hazards of morphologic origin include steep gradients; the morphologic features suggest the occurrence of mass-wasting instabilities. Hazards of sedimentary origin also includes the occurrence of slope instability processes in form of single slides and a great variety of erosive and depositional gravity flows (debris and turbidity flows). Hazards of tectonic and seismic origin are important because the sinking area straddles the Calida Bank which is a structural seamount with a moderate tectonic activity that results in a latent seismicity of low to moderate magnitude. The interaction of these factors leads to consider to the risk as medium, and the degree of exposure of the bow and stern as high. Several general and specific recommendations are made in order to increase the geological and geophysics knowledgement in the Prestige sinking area and Spanish continental margins and deep sea areas. These recommendations also should be used to elaborate the options for reducing the hazard and loss.

Areas with gas accumulations and gas escapes have been mapped in two rías from Galicia (Ría de Pontevedra and Arousa) and compared with already published data from the Rías de Vigo (García-Gil et al. 2002 ) and Muros-Noia (Magariños-Álvarez et al. accepted). Calculations indicate different areas of gas-bearing sediments for each ría. Quantitative data from acoustic plume studies and pockmark densities in the seepage areas were also obtained. In terms of the area of gas-bearing sediment and seeping activity, the Ría de Arousa is found to be the most important from a quantitative point of view. Comparison of the locations of the gas accumulations with grain size distributions of sediments reveals a spatial coincidence with finer surface sediments that are mainly muds.