GeoContextsé    çGeology    M2 Speleogenesisê    Hydrologyè

Cave Development Around Vega Huerta

The limestones of the Frontal Nappe Unit rest on the rocks of the Pisuerga-Carrion Province. These are largely black shales, sandstones and conglomerates and provide an impermeable basement to the limestones. The Rio Cares has cut completely through the Frontal Nappe and now flows some 300 metres below the base of the limestones. Therefore vadose conditions must have existed throughout the Frontal Nappe for a considerable period. At the present time, water draining to the base of the Frontal Nappe can be expected to flow down the dip of the basal thrust, approximately towards 010°.

Cave development started with the exposure of the Carboniferous limestones from beneath the post-Variscan sedimentary cover. Erosion of this cover would have both accompanied and followed the uplift of the Picos de Europa. Major cave systems probably developed while the post-Variscan cover supported large drainage catchments and concentrated water into a few sinks. As the cover was eroded however, an increasing number of smaller catchments would have developed as new fractures were exploited. Many of the existing sinks and cave passages would have been abandoned.

The glaciation of the Picos de Europa during the quaternary had a number of important affects.

• Many established depressions were undoubtedly choked with debris.
• Cave systems were probably formed by glacial melt water sinking into the limestones.
• The glaciers provided large catchments and concentrated water into certain sinks.

Since the retreat of the glaciers the surface of the Frontal Nappe has probably changed very little except that most of the larger shafts have gradually choked with scree. At present there are hundreds of shafts developed in the surface of the Frontal Nappe but very few lead to negotiable cave passages. The large number of fractures means that each one has only a small catchment area, often receiving water only from the snow-plug within it. These flows are insufficient to keep cave passages clear of the debris loosened by freeze-thaw action in the winter months. The entrances to the ‘cover systems’ are almost certainly eroded or buried, although deeper passages may have been preserved. The major cave systems discovered to date are usually those with entrance passages which have been created or enlarged by melt water and which have then escaped infill because of a fortunate location or because they have high-level routes over the blockages created by debris from the surface.

GeoContextsé    çGeology    M2 Speleogenesisê    Hydrologyè

Bibliography

Geology and speleogenesis of the M2 Cave System, Western Massif, Picos de Europa, Northern Spain, Kevin J Senior (YUCPC), Cave Science, 1987, 14, 3, 93-104, in English. PDF (5MB)