Large gypsum nodules in the Paleogene and Neogene evaporites of Spain: distribution and palaeogeographic significance

Abstract

Gypsiferous units occur along the margins of some Paleogene and Neogene basins in Spain. These units accumulated in shallow saline lakes of low ionic concentration. Other gypsiferous facies constitute outer rings of the thick, highly-saline evaporite formations that occupy the central parts of the basins. In some of these gypsum units and rings, large nodules (from 0.5 m to several metres across) of secondary gypsum that originated as replacive or displacive nodular anhydrite are present. Although these occurrences usually show a stratiform arrangement, vertical geometries are observed locally, suggesting ascending circulation of anhydritizing flows. The common characteristics of the large gypsum nodules in the various occurrences indicate that the precursor anhydrite formed in burial conditions from shallow to moderate depths. A few occurrences of large, secondary gypsum nodules in the gypsum units are linked to deep faults or diapiric structures.