Morphological characterization of pockmarks in the surrounding areas of Capbreton canyon

Abstract

Capbreton canyon runs in an east-west direction parallel to the northern Spanish coast, in the Bay of Biscay, for more than 300 km reaching depths of more than 3,000 m. Within the framework of the European LIFE IP INTEMARES project, some marine areas are being studied and could be protected due to their importance for habitats, according to the Natura 2000 Network. One of these areas is a sector of the Capbreton submarine canyon system, off the coast of the Basque Country. During two oceanographic surveys carried out in the framework of this project, bathymetric, backscatter and high-resolution parametric data were acquired, as well as direct samples and video transects of the seabed. A geomorphological characterization of this sector is being carried out. The area includes a section of the main canyon with meandering morphology, where numerous terraces and erosive escarpments are observed. On its southern slope, there are tributary channels perpendicular to the Spanish coast, which intersect the slope and define intertributary platforms with extensive fields of pockmarks. These morphologies are circular or elliptical depressions related to submarine fluid leaks inside the sedimentary deposits under overpressure. Around 3000 pockmarks have been mapped in the area with automatic and manual techniques. They are located between 400 and 1000 m deep and have diameters from 30 to 900 m with depths ranging from a few meters to 80 m. Pockmarks can be single or have multiple sinks. In certain sectors, there are preferential alignments, which could respond to a tectonic control and a gradation in depth according to areas of fracturing. Connections between these morphologies are also frequent, giving rise to linear incisions or small detachments, suggesting a relationship between pockmarks and areas with unstable slopes. The origin of the pockmarks could be after the formation of these linear incisions, caused by fluid leakage in sediment-filled canyons. In the seismic profiles acquired, the characteristic reflectors of this type of fluid escape morphologies are observed, as well as paleo-pockmarks or pockmarks inside the sedimentary deposits that have not emerged.