Resumen de Evolución hidroquímica de un sistema lacustre endorreico de pequeñas dimensiones: la cuenca oligocena de As Pontes
A small sized (12 km2) Late Oligocene lacustrine system developed in the strike-slip A s Pontes basin, N W Spain. Spread of the local watershed increased the water input into the basin, producing a major change from dominant ephemeral, shallow lacustrine-palustrine conditions to widespread perennial, deeper facies. The lower shallow facies are made up by mudstone-carbonate cycles whith partialy dolomitized carbonates. The mudstones were deposited under holomictic conditions and yielded athalassic fauna and Mg-rich clays, which indicate oligosaline-mesosaline water.
Deeper facies are microlaminated clay-carbonate, organic-rich rhythmites deposited under meromictic conditions. Changes in the Mg and 8,sO content of the primary carbonates in these laminae (high and low magnesium calcite) record short-term, probably seasonal solute increases, which were punctuated by dilution episodes. The covariant 8lsO - 8UC trends point to hydrologically closed conditions, whereas the isotopic 8wO values suggest a water reservoir small enough to allow rapid isotopic fractionation of the meteoric waters. These geochemical signatures correspond well with the palaeogeographic reconstructions of the lake system based on stratigraphic and structural data.
