Paleosols and Alunite Silcretes in Continental Cenozoic of Western Portugal

Abstract

Two different origins are proposed for fossil silcretes in continental Cenozoic sedimentary rocks of Western Portugal. These soils developed in more or less the same parent materials. The lower silcrete, 1 to 10 m thick, is a consistent indicator at the top of an upward-fining member and shows clear evidence of weathering processes: the decay of illite and its replacement by kaolinite, the redistribution and concentration of iron oxides, the impregnation of the sediment by silica minerals (opal-CT with the optical properties of quartzine under the petrographic microscope), and a late concentration of alunite rhombs along cracks in the upper part of the silcrete. This silcrete resulted from prolonged exposure of sediment to soil processes in a well-drained environment. The silcretes interbedded in a younger member are locally silicified layers at the top of upward-fining units. Silica minerals (opal-CT) precipitated where palygorskite also formed in abundance. Alkaline ponds are postulated by the numerous slickensides, due to shrinkage and swelling of clay minerals, indicate temporary subaerial exposure of the sediments and later silicification. Many rhombic boxworks in these silcretes could be the expression of a previous alunite precipitation, similar to alunite concretions at the top of this member. These silcretes in the younger member formed by repetitive processes acting in local swampy environments under a relatively dry climate. Alunite, which is abundant at the top of the lower silcrete, must not be considered as a final product of weathering. It is probably a later mineral phase introduced when the overlying beds were laid down.