OUCC Proceedings 9 (1979)

Geological Notes

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by Kev Senior

These notes are the results of observations made during the expedition, although no detailed geological study was undertaken.

The Picos de Cornion consists almost exclusively of limestone which outcrops continuously across a vertical range of some 2000m in certain areas. The region is therefore of great interest to speleologists. The limestones are usually massively bedded, but bedding planes are often difficult to identify because the rocks have been folded, faulted and intensively jointed. The structural geology is complex and would require much work to resolve in detail. A lack of suitable marker horizons and of macrofossils make structural analysis difficult. The structure is the result of superimposition of Pyreneean orogenic deformation on Cantabrian deformation. The latter is of Carboniferous age and is dominated in this region by Asturic Phase at 290 to 295m.y.

Most of the limestones are unfossiliferous calcarenites, calcisiltites, and calcilutites. There seems to have been some re-crystallisation. Beds of crinoidal limestone outcrop occasionally, good examples being seen on the path down to El Hoyo la Madre. A bedding surface is visible which is crowded with crinoid fragments such that they make up the framework of the rock. Some ossicles reach 2.5cm diameter, and some have been ankeritised leaving others, and the interstitial matrix, unaltered.

Some of the limestones have been ankeritised, or dolomitised, on a larger scale. These rocks are visible as rusty brown outcrops on the mountain sides, covering areas from 1m to 1km. There does not seem to be much spatial pattern to these outcrops, although some are clearly veins.

Approximately 2km SW of Ario the limestones are mainly finely laminated calcisiltites and calcilutites. The laminŠ vary in colour from white through black, and the rocks are made even more attractive by polyphase calcite veining which pervades the rocks adjacent to fault zones. The effect is enhanced by glacial polishing of outcrops. Small folds predate the veining and appear to be soft sediment slump structures.

Extremely coarse calcirudites outcrop at several localities. These extraordinary rocks are composed of limestone pebbles, and occasionally, calcareous shale. The clasts each some 0.25m maximum length, and the rocks are unsorted in respect of both size and shape. More clasts show some degree of rounding, and the observation that some are very rounded means that sedimentary origin must be favoured over a tectonic one. These rocks may be similar to those described by NJ Hancock in Cueva de Tinganon and Cueva Negra (OUCC Proceedings 7), although they were described as fault breccias. Fault breccias are common features of the caves of the Picos de Cornion, but are clearly identified by their angular clasts and, usually, a calcite cement. The calcirudites are possible a submarine slide deposit.

At one locality, rocks which appear to be turbidites were fund. The outcrop is in the stream near the path from Los Lagos to Ario, about 2km from Ercina. The rocks are easily recognised because they weather to a brown colour. Boulders in the stream illustrate the Bouma Sequence, including calcilutite lithoclasts in a graded calcarenite A horizon. Cross lamination in the C horizon is not well developed.

The calcirudites and turbidites suggest that limestone shelf environment was subject to tectonism before the Asturic Phase deformation. The early syn-depositional tectonism is probably related to the Bretonic and Sudetic phases. The Asturic phase deformation caused decollement folding of the limestones. These large scale features were not seen, but large recumbent folds verging N were visible in the mountain sides. The recumbent folds are probably only one of several folding episodes, but more detailed work will be required to erect a deformation chronology.

Faulting is certainly polyphase and much fault movement post-dates folding. There are some low angle faults, probably thrusts, which represent a further stage in deformation beyond the recumbent folds. Many of the faults are visible in detail because all the caves visited are strongly controlled by fault zones. Thick fault breccias accompany the faults and even calcite veins are brecciated and then recemented by further calcite intrusion. Pozu del Cantu del Hombre is developed along a mineralised fault zone. The first mineralisation was of haematite and quartz, the latter appearing particularly clear and attractive in the cave. No large crystals were found but polished pebbles abound throughout the streamway and are especially common in Mess Hall. These minerals were brecciated and then caught up in an extraordinarily massive calcite intrusion. The whole of Pillar Chamber and most of Surprise Shaft are in calcite. There is reason to believe that the whole cave is developed within the mineral vein as weathering the first ledge in Surprise Shaft has revealed the cleavages of the calcite and shows that about a square metre of the wall consists of a single calcite crystal. Jointing is irregularly developed but at some locations at least calcite crystal. Jointing is irregularly developed but at some locations at least three conjugate joint sets can be identified. These testify to the repeated deformation of the limestone in the area.