Silt-sized sediments and gypsum on surface formations in the Ebro Basin: a disambiguation of the term “gypsiferous silts"

The term “gypsiferous silts” has been used since the 1960s by many authors, but it has relied on unclear, am-biguous criteria that attributed aeolian, colluvial or alluvial origins to these materials. The aim of this paper is to clarify the use of the term “gypsiferous silts” applied to silt-sized, gypsum-bearing sediments on surface forma-tions in the Ebro Basin, using published and unpublished information. We distinguished three types of materials with very different characteristics that had all been attributed to the general term “gypsiferous silts” in previously published work: i) gypsiferous silts sensu stricto, ii) loess and iii) flour-like gypsum. The recommendations of our study are that the first two terms should be applied to gypsum-containing sediments located in varied topographic positions with calcium carbonate contents up to 40-50% and coarse silty to fine sandy textures. Loess normally appears as thick outcrops (up to 12m) on slopes leeward from the main wind direction, with a limited pedogenesis consisting of a partial carbonate mobilisation and gypsum contents between 0 and 30%. Loess is the only category with an aeolian origin. The materials in the category “gypsiferous silts” sensu stricto are mainly colluvial-alluvial. The third term (flour-like gypsum) should be used to refer to materials on surface formations consisting of almost 100% silt- to fine sand size lenticular gypsum. These materials derive from on-site weathering of gyprock or by precipitation from calcium- and sulphate-saturated groundwater and surficial waters and show a variable spatial distribution: along footslopes, outcropping as metric surface formations, interstratified between limestone layers or as generalized karstified subsurface accumulations.The proposed classification, based on morphological and physico-chemical characteristics, establishes a neces-sary distinction among the three types of materials, since they differ in their properties and geotechnical behaviour relevant for soil management and land use. Using the proposed classification will allow their distinct characteristics can be taken into account when performing geological or soil surveys in this and similar arid regions.