Understanding sediment dynamics of two large submarine valleys from seafloor data: Blanes and La Fonera canyons, northwestern Mediterranean Sea
The studied area appears as a major place for dense shelf water (DSW) formation and export towards the deep basin with Blanes and La Fonera canyons as most efficient pathways. These two canyons are so deeply indented on the continental shelf and slope of the northern Catalan margin (Western Mediterranean Sea) that their heads significantly reduce the width of the shelf and interfere with coast and contour parallel sediment transport patterns. This study presents the results of the analysis of swath bathymetry data and parametric seismic reflection profiles showing the detailed morphology and uppermost sedimentary structure of Blanes and La Fonera canyons, with the focus on their heads and adjacent upper courses and shelf. The main aim is to understand their functioning and assess how they influence and respond to the hydrosedimentary processes active in the study area.
The N–S oriented Blanes canyon head, whose shortest distance to shore is only 4 km, extends along 21.7 km from 70 to 1300 m water depth with an average rim to rim width of 8 km and a canyon floor width up to 750 m. La Fonera canyon head extends along 28 km from 60 to 1700 m water depth. Rim to rim and canyon floor widths are up to 7 km and up to 700 m, respectively. The canyon head follows a general WNW–ESE trending course in its deeper part, but trends N–S in its shallower course formed by Cap Begur branch. South of it, the Illa Negra branch trends NW–SE and its tip, located at 60 m water depth and 800 m from the coastline, intersects the littoral sedimentary prism. Backscatter data show high reflectivity into both Blanes and La Fonera canyon floors as well as on tributary gullies, which at these locations is indicative of coarse sediment.
The seafloor and subseafloor observations here presented are explained and best understood, from the sedimentological viewpoint, by the interplay of event-driven DSW flows, permanent mesoscale circulation and storm action. While DSW flows from the Gulf of Lion and the shelf area around the studied canyons are the most dynamic agent in terms of sediment transport and seafloor shaping, the Northern Current ensures background sedimentation of fines, and coastal storms promote episodic entries through canyon heads and upper course rims. Canyon wall morphology and sediment draping respond to the currentward or leeward position of each wall with respect to the main, southward moving water flows in the area, either episodic or permanent, that are DSW and the Northern Current. Other relevant morphosedimentary features, such as a 40 km long, southbound subdued channel on the Roses outer shelf that originates off Cap de Creus promontory to finally feed Cap Begur branch of La Fonera canyon and a contouritic ridge and moat attached to the northern wall of the canyon are interpreted in terms of DSW flows.
A comprehensive model of the sedimentary functioning of the canyoned north Catalan margin that integrates seafloor information and background data on dynamic processes is presented, which could be of application to other continental margins worldwide. Such an integrative view is eased by the enormous, unprecedented multidisciplinary research effort carried out in the study area over the last three decades.
Center for Coastal and Ocean Mapping
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Lastras, G., Canals, M., Amblas, D., Lavoie, C., Church, I., De Mol, B., Duran, R., Calafat, A.M., Hughes-Clarke, J.E., Smith, C.J., Heussner, S., and "Euroleón" cruise shipboard party, 2011. Understanding sediment dynamics of two large submarine valleys from seafloor data: Blanes and La Fonera canyons, northwestern Mediterranean Sea. Marine Geology, 280: 20-39. doi: 10.1016/j.margeo.2010.11.005.