Shallow slides and pockmark swarms in the Eivissa Channel, western Mediterranean Sea

Four slides, the Ana, Joan, Nuna and Jersi slides, have been imaged on the seafloor along the Balearic margin of the Eivissa Channel in the western Mediterranean. They have areas of up to 16 km(2) and occur in water depths ranging between 600 and 900 m. Volumes range from 0.14 to 0.4 km(3). Their headwall scarps, internal architecture and associated deposits are characterized using a combination of swath bathymetry data and very high-resolution seismic reflection profiles. In general, they show horseshoe-shaped headwall scarps and distinct depositional lobes with positive relief. Internally, the slide bodies are mostly composed of transparent seismic facies. Chaotic facies are observed at the toe of some of the slides, and blocks of coherent stratified facies embedded in the slide deposit have also been identified. The reflection profiles demonstrate that the four slides share the same slip horizon, which corresponds to a distinctive continuous, high-amplitude reflector. Furthermore, the geometry of the headwall scars of the Nuna and Ana slides reveals evidence of pockmarks, and fluid escape features are also present further upslope. This indicates a possible link between fluid escape features and destabilization of the upper sediment layers. In addition, these well-characterized slides demonstrate the pitfalls associated with calculating the volume of the slide masses using headwall scarp heights and the assumed preslide seafloor topography in the absence of seismic data. The internal structure also demonstrates that transport distances within the slides are generally low and poorly characterized by headwall scarp to slide toe lengths.