Gypsum karstification induced subsidence: Effects on alluvial systems and derived geohazards (Calatayud Graben, Iberian range, Spain)

The karstification of the tertiary gypsum formations in the Calatayud Graben has given rise to synsedimentary and postsedimentary subsidence phenomena in both the Neogene sediments of the endorheic graben fill and in the Quaternary alluvial sediments deposited under exorheic conditions. In the so-called Maluenda and Perejiles areas, covering 4.4 and 12 km(2) respectively, the Neogene sedimentary units stratigraphically above the gypsum have subsided due to the karstification of the underlying evaporites. The maximum subsidence in both areas has reached at least 200 m. The Jalon-Jiloca-Perejiles alluvial system has been affected by karstic subsidence during its Quaternary evolution. Ten stepped alluvial levels (pediment-terrace) have been identified. As a consequence of synsedimentary karstic subsidence the deposit of an alluvial level can be locally thickened, changing in a short distance from less than 10 m to more than 100 in in thickness. Thus, the deposit of an individual terrace can be locally superimposed on the deposits of older terraces. The palaeokarst recorded in the sediments of the different alluvial levels shows: how the karstic subsidence has controlled the dynamics of the sedimentary system. The alluvial karstic subsidence is currently active in the flood plains affecting fluvial channels, buildings, communication ways, irrigation ditches and the Calatayud gypsum scarp.