OUCC Proceedings 13 (1991)Scientific work in the Picos de Europa |
Proc. 13 Contents. |
A geological section through Pozu del Ojo de la Bruja and Sistema del Xitu was constructed in order to illustrate the relationship of the caves to the geological structure. The section was drawn projected onto a vertical plane perpendicular to thestrike of bedding (030° ). The section was used to demonstrate the geological controls on passage development. A summary of the geology of the area is given first to aid understanding of the cross-section.
The limestone massif of the Picos de Europa is made up of deformed Carboniferous limestones (circa 300 million years old), above a thin sequence of Devonian sandstone. The limestone sequence is repeated many times from south to north across the Picos de Europa by thrust faults. These are brittle fracture planes that cut across bedding at low angles. They commonly have ramp-flat geometries, being parallel to the bedding on the flats and cutting up through the beds at up to 30° or 40° on the ramps. Movement across these fractures, from north to south in the Picos, resulted in older, structurally lower rocks, being placed on top of younger, structurally higher rocks. Because the fractures are cut up through the sequence, the dip of beds in higher thrust sheets is greater than that in lower thrust sheets. In the Picos de Europa, this results in an increase in dip from gentle northerly dips in the south of the Picos to near vertical dips and even locally upsidedown beds in the vicinity of Ario (Fig. 1). The thrust faults in the Picos developed as part of a complex compression of the crust in Cantabria at the end of the Carboniferous (circa 280 million years ago). This was part of the important Hercynian mountain building event that affected a belt of rocks running through Europe and North America.
The Carboniferous Limestones of the Picos de Europa can be divided into six recognisable units which are described from oldest to youngest (see Fig. 2):
This basal unit is a thin, red nodular limestone that was deposited over a considerable period of geological time. It is not seen high in the western massif but does occur in east-west bands in the Cares Gorge, near the base of the thrust sheets.
The Griotte Limestone is overlain by the dark, banded, cherty limestones of the Barcaliente Formation. This occurs at the base of the thrust sheets where the Griotte Limestone has been faulted out in the western massif.
The Valdeteja Formation is a grey massive limestone with occasional chert beds. The boundary between the Barcaliente and the Valdeteja Formations is transitional. The Valdeteja Formation outcrops at the top of Jultayu and in much of the area around Ario.
The Lower Picos de Europa Formation consists of thinly bedded, occasionally bioclastic limestone. In places, the limestone contains abundant chert beds, 1-10cm thick. This formation is commonly encountered in the deeper regions of the caves in the Verdeluenga-Punta Gregoriana area.
The Upper Picos de Europa Formation consists of more massive white or pink limestones. These are locally bioclastic. The entrance series to the Verdeluenga- Punta Gregoriana caves are developed in this Formation.
The youngest limestone formation consists of further tabular limestones. This does not outcrop in the current area of interest.
The cross-section (Fig. 3) has been constructed using the geological map of Farias ([1], Fig. 1) together with observations made underground. The section shows that Pozu del Ojo de la Bruja initially drops steeply through Valdeteja Formation limestone before penetrating the dark, finely laminated Barcaliente Formation in Just Awesome. The streamway and the London Underground start approximatedly parallel to the bedding in Barcaliente Formation but soon bend to north-east and cross into the overlying Valdeteja Formation in the vicinity of Choke Arnold. Downstream of the 1989 Camp, Pozu del Ojo de la Bruja crosses a major thrust fault and reenters the Barcaliente Formation. From here downstream, the bedding dips back to the south and the beds are inverted. The cave crosses back into the Valdeteja Formation upstream of Postman Pat and Choke Egbert is just short of another major thrust fault, above which the Valdeteja Formation dips to the north again near the top of the Canal de Trea.
Sistema del Xitu is entirely developed within the north dipping fault block to the north of Choke Egbert.
The extensive, deep, near-horizontal passages in Pozu del Ojo de la Bruja show two distinct passage trends, Nortwest-Southeast and Northeast-Southwest. The Northwest-Southeast passages are parallel to the local strike of bedding; some passages, eg the Picadilly Line and Postman Pat, are bedding chambers. The Northeast-Southwest passages define the dominant trend of Pozu del Ojo de la Bruja and are parallel to a well-developed vertical joint orientation. These joints, fractures across which there is no displacement, are easily seen on the surface and can even be confused with bedding where they are well-developed. The northwards tilting of Sistema del Xitu reflects the northerly dip of bedding.
[1] Farias P. (1982) La exstructura del sector central de los Picos de Europa. Trabajos Geologica. Univ. de Oviedo 12 63-72.
Phil Rose