Control sedimentológico de los procesos de alteración y rubefacción en la formación areniscas de Aldearrubia (Eoceno Superior, Salamanca)

Abstract

The present work is a contribution to the study of the alteration and rarefaction processes in the Aldearrubia Sandstone Formation whose sedimentological characteristics have been dealt with in previous works. Rarefaction is more or less generalized throughout the unit, though with important variations in intensity. It is manifested by a reddish tinge in the' scarce clay matrix, the precipitation of Fe oxyhydroxides around the grains and the direct precipitation of oxides into small pores. Alteration has led to an important replacement of feldspar grains, micas and chlorites by smectites. The findings reported reveal the existence of a perfect correlation between the alteration and rarefaction processes, both of them being maximal in the porous channel beds and minimal in the muds of the flood plain formed of paleosoils such that the greater the porosity and the greater the lateral proximity of the bed to active channels, the greater the intensity of both processes. This arrangement is anomalous with respect to similar series, where at least the strongest alteration processes are found associated with the sedimentary interruptions representing the paleosoils. Such a discrepancy is interpreted as a function of the dynamics of water fluctuations in the active channel and its interaction in the water table levels of the lateral porous beds.