The large evaporitic formation known as the Calatayud Gypsum (Lower-middle Miocene) may be subdivided into two (informal) units, lower (l.e.u.) and upper (u.e.u.), each one displaying differentiated sedimentologlc and diagenetic features. In these lacustrine units a facies change is observed from the margin (low salinity, massive lithofacies, primary gypsum preserved) to the centre (higher salinity, laminated lithofacies, secondary gypsum) of the basin. The thick l.e.u. contains calcium-sulphates and sodium-bearing sulphates (mainly glauberite), whereas the u.e.u. contains calcium-sulphates and associated carbonates. The depocenter of the u.e.u. shifted toward the SE with respect to the l.e.u. In both units, meganodular anhydrite, which generated in a sabkha environment and associated with red lutites, intercalates in the transitional zone between the central (laminated lithofacies) and the marginal (burrowed gypsum with abundant chert) zones. In the subsident Morata zone, there is a distinct cyclicity (gray lutites-laminated sulphates) in the two units; this succession exceeds a total thickness of 200 m.