Characterisation of the recent BIG'95 debris flow deposit on the Ebro margin, Western Mediterranean Sea, after a variety of seismic reflection data

• Published in: Marine geology Vol. 213; no. 1; pp. 235 - 255
• Main Authors: Lastras, G., Canals, M., Urgeles, R., De Batist, M., Calafat, A.M., Casamor, J.L.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15-12-2004
• Subjects: debris flow; Marine; seismic reflection profiling; slope stability; swath bathymetry; Western Mediterranean; swath bathymetry; Western Mediterranean; debris flow; seismic reflection profiling; slope stability
• ISSN: 0025-3227; 1872-6151
• DOI: 10.1016/j.margeo.2004.10.008

Swath bathymetry and backscatter data, side-scan sonographs, high-resolution (HR) and very high resolution (VHR) reflection seismic profiles, and sediment cores reveal the complexity of the seafloor and subseafloor features resulting from a large debris flow event that affected the Ebro continental slope and base-of-slope at the beginning of the Holocene. The BIG'95 debris flow, as it has been named, disturbed more than 2200 km
2, including a 26-km
3 deposit of remobilised sediment covering ∼2000 km
2. Swath bathymetry and backscatter imagery allow to distinguish four main areas within the debris flow: the source area, the proximal depositional area, the intermediate blocky depositional area and the distal depositional area. In the source area, a sinuous headwall scar, 20 km long and up to 200 m high, and several other secondary scars have been identified. Sediment released from the source area flowed southeastwards to the proximal depositional area, which is the main depocentre of the BIG'95 debris flow deposit, with accumulations over 90 m thick. Large slabs of sediment detached from this area and from the headwall scar and moved southeastwards embedded in a more mobile matrix, thus forming the intermediate blocky depositional area. Only the looser matrix reached the distal depositional area, finally freezing in and partially burying the Valencia Channel, a mid-ocean type channel. Digitisation of VHR seismic reflection profiles, where the debris flow deposit is shown as a body of mainly transparent seismic facies, has allowed the construction of isobath and isopach maps showing the thickness distribution of the deposit and its interplay with preexisting canyons and channels. Dating of two sediment cores gives a minimum age of 11,000 calendar years BP for the BIG'95 debris flow. A set of triggering mechanisms, including seismicity and oversteepening of the slope due to the existence of a volcanic structure underneath the main headwall, is invoked.