Sediment distribution patterns on the Galicia-Minho continental shelf

A sedimentological and bathymetric study of the Minho-Galicia Shelf shows a strong contrast between a southern shelf region with a thin partially relict cover of sands and gravel, and a northern region where fine-grained sediments predominate. This contrast is explained through differences in the sediment supply, the oceanographic environment (storms and ocean currents) and the morphology of the shelf which results from its underlying tectonic framework. Most sediment is supplied to the Galicia-Minho Shelf by river discharges onto the Minho Shelf, particularly that from the Douro River. In the northern part of the shelf the Galician Rias act as sediment traps rather than sediment suppliers. The bulk of the sediment washes out of the rivers during episodic storm events. While most of the coarse sediments remain deposited close to the coast. the fine-grained material is exported to the outer areas of the shelf. Subsequently, coarse sediments close to the coast are transported southwards by the littoral drift. Whereas the fine-grained material is frequently resuspended through the action of the large swells who influence reaches deep into the water. This frequent resuspension has a long-term sorting effect on the sediments. Furthermore. resuspended sediments on the middle and outer regions of the shelf are transported northwards by a poleward flowing bottom current. As a consequence of the differential transport of coarse sediments to the south. and of the fine-grained sediments to the north, the outer reaches of the Minho Shelf are relatively poor in recent sediments. In many areas relict sediments as well as features associated with ancient coastlines and river mouths. still appear as seabed features, In contrast, the northern regions of the shelf are covered by a thin veneer of fine-grained material that smooth other most of these fossil features. The fine-grained sediment fractions (mostly very fine sands to coarse silts) are deposited in two large mud patches. the Douro and the Galicia Mud Patches. which are situated at water depths of around 100-120 m. These two mud patches are both controlled by the local hydrodynamics and morphology. The Beiral de Viana. to the west of the Douro Mud Patch is a plateau. up to 20 m high lying parallel to the shelf-break and is a morphological expression of an underlying horst system. This plateau acts as a barrier that prevents the drift of some of the fine-grained material to the west, out over the shelf-break and the continental slope. The Galicia Mud Patch is situated on the eastern part of the Galician Shelf to the north of the Douro Mud Patch. It is situated near the extension of the Porto-Tomar fault, which results in the shelf being usually steep in this region, down to a depth of about 100 in. West of this area the slope is much more gentle. Northward transport of the sediment is strongly reduced by the E-W trending outcrops of plutonic and metamorphic rocks. (C) 2002 Published by Elsevier Science Ltd.