Subject 1: INTRODUCTION TO ENVIRONMENTAL RESOURCES IN THE BASQUE COUNTRY

Module 1A. GENERAL CONDITIONS OF THE BASQUE COUNTRY: SUBSTRATE AND CLIMATE

SHAPING UP THE BASQUE COASTAL ZONE: GEOLOGICAL CONTROLS

Victoriano PUJALTE Dept. Estratigrafía y Paleontología. Universidad del País Vasco

The Basque coast is, predominantly, a cliffed coast. Such coasts occur at rocky shorelines because of the destructive action of waves, especially during storms. Larges waves hitting the coast exert enormous pressures at the base of the cliffs, breaking and removing fragments of rocks. Such erosion first cut back an indentation, or “notch”, at the at the bottom of the cliff, and eventually result in the collapse of the rocks above, thus creating a vertical or near vertical cliff- face. Reiteration of this process causes cliff retreat and generates an erosional, near horizontal surface known as wave-cut platform or, to use the local name, “rasas”. Present-day rasas are best observed at low tide. Ancient, uplifted rasas can also be seen, attesting to either a former higher position of the sea-level, a tectonic elevation of the coast or a combination of both factors.

Inspection of the Basque coast (or of any cliffed coast elsewhere) readily reveals that its outline is highly irregular, with capes, promontories or headlands alternating with bays and estuaries. This implies that the rate of erosion varies greatly along the coast or, in other words, that some parts of the coast are easier to erode than others. Such differential erosion is controlled by two main factors:

(a) Type of rocks. As explained in a previous talk, rocks of the Basque Country are predominantly of sedimentary origin, both of siliciclastic (conglomerates, sandstones, mudstones) and carbonate nature (limestones, marls). A small proportion of igneous rocks also exist. Igneous rocks, conglomerates, sandstones and limestones (especially when they are massive) are very resistant to erosion, whereas mudstones and marls are soft and easily removed. Besides, limestones and marls are not only mechanically eroded, but can be dissolved as well.

(b) Position of rocks relatively to the sea. One of the most distinctive features of sedimentary rocks is that they are stratified, that is, arranged in beds or layers called strata. Strata are originally laid down horizontally but, if are later subjected to tectonic stresses, they may be Subject 1: INTRODUCTION TO ENVIRONMENTAL RESOURCES IN THE BASQUE COUNTRY

Module 1A. GENERAL CONDITIONS OF THE BASQUE COUNTRY: SUBSTRATE AND CLIMATE tilted or brought to a vertical position. Strata can thus be found sloping away or inclined towards the sea, placed vertically parallel or perpendicular to the shore, or at any other position intermediate from those.

Others factors influencing the rate of coastal erosion are whether the rocks are homogeneous or made up of alternating lithologies, the wholeness or splintered state of the rocks and, of course, the relative intensity of wave action. In this talk, examples showing the effects of the above controls, notably of (a) and (b), will be demonstrated.

A minor portion of the Basque coast (about 10%) is fronted by sandy beaches, the location of which is also largely controlled by geological factors. Some of the beaches are placed (or used to be placed) at the mouth of estuaries, the position and shape of which being determined by the underlying geology. The Bilbao estuary, for instance, occur in a zone where the coast is dissected by a major fracture (the Bilbao fault), which splintered the country rocks and made them much less resistant to erosion. Likewise, the Urdaibai estuary is located at the core of a major estructure (a “diapiric anticline”) in which soft easily eroded Upper Triassic clays and evaporites were brought to the surface. Most of the Basque beaches, however, occur at the head of bays (and are consequently named “bay-head beaches”). In plant view, they are characterized by a crescent-shaped profile, which closely match the contour of the crests of the approaching waves, a contour in turn dictated by the shape of the surrounding cliffed areas.

The underlying geological control in the shaping of the Basque coast also mean that care must be taken when tampering with coastal features: this will be illustrated with examples of undesirable side effects that followed man-made actions in or near the shoreline.