Weathering of host rock and corrosion over speleothems in Castañar cave, Spain: an example of a complex meteoric environment

Abstract

In Castañar cave, weathering processes have been identified on the host rock and speleothems. The result of these processes over speleothems, usually called corrosion, causes the loss of brightness and the formation of a powdery matt outer band. This band is composed by crystals <4 µm that cover the surface of the speleothems and changes the original texture. Corrosion is caused by partial dissolution of the aragonite and calcite that form the speleothems and results in the formation of these small crystals which are not washed away and remain in situ. The presence of similar bands inside and on the surface of the speleothems indicates that corrosion occurs not only at present time, but it has occurred several times interrupting the development of the speleothems. Castañar cave host rock consists of beds of Fe-rich dolomite and magnesite interbedded with greywackes and shales. Weathering of these rocks produces a clay-rich residue of intense red colour formed by quartz, clay minerals and iron oxides and hydroxides. This residue was formed by in situ alteration of the host rock either by dissolution of the dolomite Fe-rich (insoluble residue) or by hydrolysis of some components of greywackes and shales. In touristic caves, the alteration of both speleothems and host rock has usually been related to the acidification of the atmospheric moisture caused by CO2 from the breath of visitors. However, the presence of corrosion lines inside and on the surface of the speleothems and the fact that some of the non-visited galleries of the cave also present alteration features, indicates that this process did also occurred in past times and hence is not related to the presence of visitors. These weathering processes accounted for the outstanding chromatic contrast of the interior of the cave and show that the growth and formation of speleothems cannot be studied without taking weathering into account.