Rock-controlled coastal landforms in the flysch-dominated Basque Coast UNESCO Global Geopark

Abstract

The approximately 30-km-long coastline of Bay of Biscay within the boundaries of the Basque Coast UNESCO Global Geopark is primarily erosional, with vertical cliffs and steep slopes more than 100 m high abruptly giving way to abrasion platforms. Minor rock coast landforms include notches, marine caves, potholes and joint-aligned clefts within abrasion platforms, and boulder accumulations. The coastal morphology has developed in a thick flysch succession of Cretaceous to Eocene age. In the western section, the flysch is dominated by grey shales and greywackes, and is known as the Black flysch of Lower Cretaceous age. In the eastern sector, separated from the western one by the Andutz Fault, the flysch is calcareous and dominated by rhythmic alternations of shales, marls and limestones. Moreover, the strike of the strata is roughly parallel to the coast in the Black flysch, whereas it is perpendicular or oblique in the Calcareous flysch. The dip is almost invariably towards the sea. These lithological and structural differences are reflected in the inventory of coastal landforms, with cliffs and wave-cut platforms much better developed in the Calcareous flysch. Coastal landslides are ubiquitous in the Calcareous flysch, facilitated by the presence of steeply dipping bedding planes, so that sliding is mostly translational. Numerous outcrops provide evidence of distortion and brittle deformation of flysch strata due to landsliding.