Evaporative systems and diagenetic patterns in the Calatayud Basin (Miocene, central Spain)

This paper concerns the evaporite units, depositional systems, cyclicity, d iagenetic products and anhydritization patterns of the Calatayud Basin (non -marine, Miocene, central Spain). In outcrop, the sulphate minerals of thes e shallow lacustrine evaporites consist of primary and secondary gypsum, th e latter originating from the replacement of anhydrite and glauberite. In t he evaporative systems of this basin, gypsiferous marshes of low salinity c an be distinguished from central, saline lakes of higher salinity. In the g ypsiferous marsh facies, the dominant, massive, bioturbated gypsum was part ly replaced by synsedimentary chert nodules and siliceous crusts. In the sa line lake facies, either cycles of gypsiferous lutite-laminated gypsarenite or irregular alternations of laminated gypsum, nodular and banded glauberi te, thenardite and nodular anhydrite precipitated. Early replacement of par t of the glauberite by anhydrite also occurred. Episodes of subaerial expos ure are represented by: (1) pedogenic carbonates (with nodular magnesite) a nd gypsiferous crusts composed of poikilitic crystals; and (2) nodular anhy drite, which formed in a sabkha. Additionally, meganodular anhydrite occurs , which presumably precipitated from ascending, highly saline solutions. Th e timing of anhydritization was mainly controlled by the salinity of the po re solutions, and occurred from the onset of deposition to moderate burial. Locally, a thick (> 200 m) sequence of gypsum cycles developed, which was probably controlled by climatic variation. A trend of upward-decreasing sal inity is deduced from the base to the top of the evaporite succession.