INSTRUMENTATION VIEWPOINT Sessions 73 SEAFLOOR CARTOGRAPHY AND CHARACTERISATION OF THE BASQUE COUNTRY INNER CONTINENTAL SHELF Galparsoro I.; C. Hernández; M. González & A. Uriarte AZTI-Tecnalia. Marine Research Division. Herrera kaia portualdea z/g. 20110 Pasaia, Gipuzkoa. Spain.Email: igalpar- soro@pas.azti.es Keywords: Earthquake, SW Iberian Margin, turbidite, Synchroneity, interdisciplinary studie Introduction igh quality information about geomorphology and seafoor charac- teristics of the Basque continental shelf is scarce. Since the frst bathy- metric charts were performed by the Spanish Marine Hydrographic Institute in the late sixties, little research has been done. Only in the early nineties, seafoor characterisation was conducted to locate and characterise sand disposal places for beach regeneration [1], [2]. Due to a growing interest in cartography of the traditional fshing sites, studies were carried out in 2000 for the Agricultural and Fisheries Department of the Basque Government [3]. These studies show an initial approach to seafoor classifcation using the Acoustic Seafoor Discrimination System, RoxAnnTM and a sidescan sonar. In 2003 the Biodiversity Direction of the Environmental and Territory Management Department of the Basque Country agreed to establish a permanent observatory of the Natura 2000 Network in the Basque Country in order to guarantee fulflment of its objectives and its for- mulation. This situation highlighted the necessity of cartographic information from the continental shelf and the identifcation and characterisation of the most signifcant marine habitats in the Basque Country. To solve these requirements, a three year project started in 2005 with the principal aim of generating seafoor cartography and seafoor characterisation of the continental shelf, the defnition and delimitation of marine habitats and the identifcation of the main species of fora and fauna associated to each habitat type. The Basque continental shelf (NE of Spain) borders to the East with France (1º46’50’’ W) and to the West with the autonomic community of Cantabria (3º9’13’’ W). The total surface of the study area is approxi- mately 936 km2 (between 5 m and 100 m water depth) and the total length of the coast is 150 km. Bathyemtric data Bathymetric and seafoor backscatter information has been acquired using high resolution multibeam SeaBat 7125 and SeaBat 8125 sys- tems, having both equipments similar characteristics. Most of the work has being carried out with the latest SeaBat 7125 model. Its op- eration frequency is 400 kHz, produces 256 beams in 128º angle swath and up to 50 swaths per second. Beam width is 0,5º along track and 1º across track producing very small footprints which produces very high horizontal resolution Digital Elevation Models (DEM). Instead of interpolating between a series of survey tracks, dense soundings can be collected from the entire seafoor and virtually no interpolation is required to produce the details of the physiographic surface [4]. Its vertical resolution is 6 mm and it is fully operative in a range of depths between 0 m and 100 m. All these characteristics make the system fulfl the IHO’s special order requirements [5]. Apart from the multibeam echosounder, the system is composed of a gyrocompass and motion sensor Octans III, a Trimble DGPS, a sound velocity pro- fler and a surface sound velocity sensor. Bathymetric data were acquired and processed using specifc soft- ware PDS2000. Tide correction was applied using the nearest tidal gage (six stations along the Basque coast) and 1 m resolution sea- foor DEM was produced for all the continental shelf (higher reso- lution DEMs are possible to produce in shallow waters if required). Finally, the DEM was exported into ESRI grid format and integrated into a Geographic Information System (GIS) for further topographic features interpretation and analysis. Main seafoor features The Cantabrian continental shelf is characterised by its narrowness. In the Basque Country, it ranges from 7 km of Cape Matxitxako, to 20 km of the estuary of the river Oria. The Cantabrian (Basque) inner shelf is covered by a quasi continuous belt of rocks, which constitutes an extension of the rocks of the continental clifs. The continuity of this rocky substrate is interrupted regularly by the presence of river mouths, beaches, etc., further ofshore the shelf is covered by sandy sediments; these, in turn, isolate the exposed rocky areas of the sea- bed. Exposure to wave energy appears to determine the distribution of sediments. The coast consists of a succession of capes and embay- ments, which have been developed by the continuous action of the waves and rivers within the region. The resistance to erosion of the diferent materials has controlled the actual morphology. Rocky seafoor Flysch type formation of rocky substrata is recognisable in subtidal areas. A shallow and high roughness bedrock belt, associated to coastal topography is dominant. It presents approx. a 10% slope, af- ter an infexion point approximately at 32 – 37 m water depth. Further ofshore the platform again extends with a milder slope (oscillating between 1,5 % and 2 %). In this area, seafoor roughness is lower and sand patches are common between rock strata. Rippled scour depressions Ripples appear distributed along all the study area. They are well de- veloped in subtidal sandy pocket beaches or in sandy areas sheltered by rocky outcrops. The crests appear well developed and they main- tain continuity during tens of meters. They present low undulation with occasional bifurcation, refecting their wave origin. Crests are oriented mainly in NO – SE according to predominant wave direction. Rippled felds backscatter signal indicate that they are composed by coarser sand than the seabed around. Sea-level change evidences Previous studies indicate that, during the last glacial maximum, sea- level was lower than 120 m than present. The main physiographic features that evidence eustatic changes are paleochannels and abrassion platforms. The use of only multibeam data is not enough to describe paleochannels because most of them are covered by re- cent material. In cases where hydrodynamic processes are energetic enough or places where erosion processes are dominant paleochan- nels are possible to identify. In general terms, they present S - N ori- entation. Actual estuaries associated paleochannels are the largest ones identifed but other smaller channels can be also distinguished. They have continuity in land with small creeks, which indicates that in former times their water discharge was signifcantly higher. Conclusions Due to the high resolution of multibeam system, new seabed features can be now be identifed and described. This project will produce base knowledge and the results obtained will be of special interest for integrated coastal management, decision makers, habitat map- ping, description of seabed processes, and studies of climatic change and related impacts. M7