Silcretes in southern Portugal - Micromorphological characteristics and differences.
Daniela Sauer, Christine Stein, and Karl Stahr. Institute of Soil Science and Land Evaluation, University of Hohenheim, Emil-Wolff-Straße 27, 70599 Stuttgart, Germany
Many soils of our study area in southern Portugal are in part strongly cemented. Some of the cemented horizons contain lime, others are lime-free. The cement in these soils is silica. The geology of the area is dominated by plutonic and volcanic rocks (mainly diorite, gabbro, porhyr, pumice). These rocks are largely covered by Tertiary sediments. The area is characterized by a softly undulating landscape, strongly influenced by the leveling effect of both, intensive Tertiary weathering under tropical to subtropical conditions, and Tertiary sedimentation filling up the basins. The climate is typically Mediterranean with mild, humid winters and warm, dry summers. The mean annual temperature is 16.2°C and the mean precipitation 550 mm a-1 (Beja). The silcretes are particularly widespread in the Sado basin. Their thickness reaches several meters. Therefore, we interpret silcrete formation in the basin not only as a result of silica accumulation in place but as strongly determined by lateral silica input from the adjacent mountains. Since there is a water surplus in winter, lateral water and element flows within the landscape take place during several months of the year. In basin positions dissolved substances may precipitate due to evaporation or a different geochemical environment. By studying thin sections of undisturbed samples of the silcretes we found that the micromorphology of the cementations differs, so that several types of silica precipitations can be distinguished. These include microcrystalline opal, amorphous silica (opal-A) and chalcedony. In some cases also palygorskite is involved. The time of silcrete formation in Portugal is supposed to be Pliocene. Several considerations lead to this assumption: Silcrete formation seems to be favoured by a semi-arid climate with frequent wetting and drying cycles of the soil. The present climate is assumed to be too humid for intensive silcrete formation. This age estimation is supported by the fact that only a small proportion of the opal is amorphous, while most of it is microcrystalline. We assume that under the present climate only little fresh (amorphous) opal is formed and that the opal, which was precipitated during Tertiary has altered to microcrystalline opal. Another clue is that the silcretes seem to be associated with the Pliocene sediments (“Ranhas”). Therefore, the silcretes cannot be older. Regarding the climate development from Pliocene until present, the Pliocene is the most favourable period for intensive silcrete formation.