OUCC Proceedings 11 (1983)

The Geology of Pozu Jorcada Blanca

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Helen Kay

Introduction

The area of Jorcada Blanca is located within the Picos de Cornion in the western part of the Picos de Europa, northern Spain. The entire region is largely developed in limestones of Carboniferous age which experienced major earth movements during the Hercynian. These began in the Westphalian with northerly thrusting, and continued in early Stephanian times with movements corresponding to the Asturian phase. The movements resulted in comparatively little folding, but in repeated thrusting and overfolding. This gave the mountains their pronounced east-west structure and topography, and means that the same sequence of rock types outcrops repeatedly from the north to the south of the region. Although there have been a number of studies of the Jorcada Blanca area in recent years, notably by the University of Oviedo, Marquinez (1978) and Farias-Arquer (1981), these have only succeeded in mapping the area (Figure 1) and have not made any detailed lithological analysis, let alone facies analysis of the limestones of the area.

Stratigraphy

The Jorcada Blanca area is composed solely of Carboniferous limes tones of Lower Viséan to Lower Namurian age, occasionally overlain by various unconsolidated deposits of Quaternary age. Four main limestone units can be recognised (Figure 2):

Mountain Limestone

  1. Barcaliente Formation: The Barcaliente Formation consists of a 'black', fine-grained, tabular limestone within which is found abundant chert, giving rise to alternating light and dark laminations. An erosional unconformity separates the Barcaliente Formation from the stratigraphically-lower Griotte Limestone.
  2. Valdeteja Formation: The Valdeteja Formation, the upper member of the Mountain Limestone, exhibits a gradational boundary with the Barcaliente Formation. The limestones are 'black' and massively- bedded, although the beds become lighter in colour up-sequence as the chert content of the limestone decreases. Intraformational breccias can be observed at the base of the Formation and beds of bioclastic limestone can be frequently seen throughout the unit.

Picos de Europa Limestone

  1. Lower Member: The Lower Member consist of a grey, bioclastic limestone with a number of chert bands. The limestone is generally finer-grained than that of the underlying Valdeteja Formation.
  2. Upper Member: The Upper Member consists of a grey coarse-grained bioclastic limestone, with impersistent red beds. It exhibits a gradational boundary with the underlying Lower Member. The fauna of the bioclastic limestone includes foraminifera, corals, bryozoa, brachiopods, crinoids and algae.

References

Farias-Arquer, P.J., 1981. La Estructura del Sector Central de Los Picos de Europa. Unpub. thesis, Univ. Oviedo, Spain.

Marquinez, J., 1978. Estudio geolgico del sector SE de los Picos de Europa. Trabajos de Geologia 10, 295-315.