SEAFLOOR MOUNDS IN THE MUROS AND AROUSA RIAS OF NW SPAIN

The rías of Galicia are located on the passive Atlantic margin of NW Spain. Ría de Muros has an area of 125 km2, while Ría de Arousa has an area of 230 km2 with main NE-SW longitudinal axes of 12.5 km and 28 km respectively. Grids of shallow seismic, 3.5 kHz sub-bottom profiler, echosounder and sidescan sonar data acquired in the rías have been the subject of detailed interpretation allowing the mapping of shallow gas accumulations and gas escape features. For the first time we have been able to identify small seafloor mounds and larger seafloor domes in the shallower waters of the rías.

The mounds in the Ría de Muros have been identified on sidescan sonar and are typically 5m in diameter and 1m high located in water depths from 10 m to 15 m. The mounds in the Ría de Arousa have diameters ranging from 15 m to 45 m and heights that range from 1-2 m in water depths from 15 m to 20 m. These mounds have seafloor expression on the echosounder as well as on the Uniboom records. Grab sampling of one mound indicates a mud composition. The Uniboom record shows extensive acoustic blanking (3.7 km along the line) due to the presence of shallow gas that is accumulating almost at the present seafloor.

The domes appear as rounded or ovoid features on the sidescan sonar having an average diameter of 50 m but occasionally reaching up to 100 m. The height of these domes is generally between 0.5 m and 1 m. The Uniboom and 3.5 kHz seismic records show a rise of the shallow gas front just underneath these domes. Grab samples taken in the area indicate mud is the dominant sediment type.

Acknowledgements Contribution to: PGIDT00PXI30105PR, REN2000-1102 MAR and 437 IGCP Spanish Projects.