Gypsum-Rich Horizons In Saline Playa-Lakes of Monegros, Spain.

Regions with gypsiferous formations as part of the geological substrate are often characterized by gypsic soils, showing specific features. Another expression of dissolution and reprecipitation of gypsum in such setting can be the accumulation of gypsum as part of sediments and soils in local depressions. The nature of gypsum occurrences of this type were investigated for karstic basins in the Monegros region of NE Spain. In these ephemeral saline lakes, gypsum can in principle form as subaqueous sedimentary precipitates and as subaerial pedogenic accumulations. The soils/sediments contain (i) abundant gypsum within the groundmass, (ii) gypsum infillings of pores, (iii) fragmented gypsum crystals, and (iv) rare fragments of the gypsum bedrock. Gypsum crystals in the groundmass are partly lenticular but various non-lenticular forms are also common. The latter indicate sedimentary gypsum precipitation in a saline lake environment, whereas lenticular forms are observed for pedogenic, sedimentary and diagenetically modified sedimentary occurrences, whereby recognition of sedimentary lenticular gypsum is based on the presence of depositional features such as layering, grading and crystal alignment. Gypsum in pores is typically initially non-lenticular, often changing to lenticular with time. Fragmented crystals record subaqueous clastic reworking. Fragments of the gypsum bedrock, with xenotopic fabric and common anhydrite/bassanite inclusions, are encountered near the base of some profiles. In comparison with gypsum occurrences in depressions, gypsum in the surrounding soils shows an absence of non-lenticular crystal morphologies, a different type of pore-related accumulations, the presence parallel-fibrous gypsum, and a greater abundance of gypsum bedrock fragments. Differences are related to a lack of sedimentary gypsum formation, a more advanced stage pedogenic gypsum accumulation, and a great difference in availability of soil solutions.