Changes of porosity due to weathering in quartzites and slates of a Raña profile (Montes de Toledo, Central Spain)

Abstract

Ranas are alluvial fan deposits of Plio-Pleistocene age that form the piedmont platforms around the mountains in the interior of the Iberian Peninsula. They are composed of cobbles, pebbles and gravels of quartzite and some quartz, all embedded within a clayey matrix displaying striking changes in hues due to hydromorphism. Beneath these platforms, the Hercynian basement is consistently deeply weathered. In the profile of the Rana, the quartzite stones located into the clayey horizon have a weathering rind that is whitish to ochre in colour, in contrast to the dark reddish hue of those located within the leaching horizon, just below the land surface. These differences are related to changes in the physical properties (e.g., bulk density and porosity), mineralogy (presence of oxyhydroxides) and weathering processes that have taken place in the profile. Such processes have led to the corrosion and replacement of the quartz grains by the iron oxyhydroxides. The main cause is the dramatic changes in the water regime occurring in the pores at the surfaces of the quartzite stones. Due to weathering the slates outcropping beneath the Rana have undergone important release of matter (ca. 30%), together with changes in the mineral association, with a progressive reduction in the component of the unweathered slates and an increase in new minerals (smectites, kaolinite and iron oxyhydroxides) upwards.