Estructura geológica del Cerro del Castillo y formación de la Gruta de las Maravillas (Aracena)

Resumen

The Gruta de las Maravillas (Aracena, SW Spain) is located within the Aracena Metamorphic Belt that, together with the Beja-Acebuche Amphibolites, constitutes the Aracena Massif. This metamorphic belt marks the contact between two major zones of the Iberian Massif (Ossa-Morena Zone and South Portuguese Zone). This study is focused on the Cerro del Castillo hill, made up from structural bottom to top of granodiorites, marbles and quartzites, and gneisses, with a general N110oE orientation. The marbles close to the granodiorite contact include a thin band of disseminated and massif pyrite, partially transformed to Fe oxides. Marbles appear highly deformed in ductile conditions, with isoclinals fold at different scales, boudins, porphyroblasts with sigmoidal morphology and left-lateral S-C shear fabrics. Analysis of the brittle deformations indicates a NE-SW oriented maximum compression. The Gruta de las Maravillas is divided into 3 levels (located at ~650, ~665 and ~685 m.a.s.l.), the dissolution having progressed from top to bottom in different stages. This dissolution probably was favored by the presence of pyrite and Fe oxides in the host rock, which, in turn, would have provoke the acidification of the circulating water. The morphology and position of the Gruta de las Maravillas cave is controlled by the orientation of the main foliation and the pyrite-bearing band in the host marbles. Nowadays, the lower level in the cave is partially submerged ~ 10 m below the water table. The formation process of the Gruta de las Maravillas is outstanding, compared to other Spanish caves (e.g. Cuevas de Nerja or El Soplao), due to the presence of the pyrite band that accelerated the dissolution of the carbonatic rock.