The stratigraphical record and activity of evaporite dissolution subsidence in Spain

The evaporite formations tin outcrop and at shallow depth) cover an extensi ve area of the Spanish territory. These soluble sediments are found in dive rse geological domains and record a wide time span from the Triassic up to the present day. Broadly, the Mesozoic and Paleogene formations (Alpine cyc le) are affected by compressional structures, whereas the Neogene (post-oro genic) sediments remain undeformed. The subsidence caused by subsurface dissolution of the evaporites (subjacen t karst) takes place in three main types of stratigraphical settings: a) Su bsidence affecting evaporite-bearing Mesozoic and Tertiary successions (int erstratal karst); b) Subsidence in Quaternary alluvial deposits related to the exorheic evolution of the present-day fluvial systems (alluvial or mant led karst); c) Subsidence in exposed evaporites (uncovered karst). These ty pes may be represented by paleosubsidence phenomena (synsedimentary and/or postsedimentary) recognizable in the stratigraphical record, or by equivale nt currently active or modem examples with surface expression. The interstratal karstification of the Mesozoic marine evaporites and the c onsequent subsidence of the topstrata is revealed by stratiform collapse br eccias and wedge-outs in the evaporites grading into unsoluble residues. In several Tertiary basins, the sediments overlying evaporites locally show synsedimentary and/or postsedimentary subsidence structures. The dissoluti on-induced subsidence coeval to sedimentation gives place to local thickeni ngs in basin-like structures with convergent dips and cumulative wedge out systems. This sinking process controls the generation of depositional envir onments and lithofacies distribution. The postsedimentary subsidence produc es a great variety of gravitational deformations in the Tertiary supra-evap oritic units including both ductile and brittle structures (flexures, synfo rms, fractures, collapse and brecciation). The Quaternary fluvial terrace deposits on evaporite sediments show anomalous t hickenings (> 150 m) caused by a dissolution-induced subsidence process in the alluvial plain which is balanced by alluvial aggradation. The complex s pace and time evolution pattern of the paleosubsidence gives place to intri cate and anarchical structures in the alluvium which may be erroneously int erpreted as pure tectonic deformations. The current subsidence and generati on of sinkholes due to suballuvial karstification constitutes a geohazard w hich affects to large densely populated areas endangering human safety and posing limitations to the development. An outstanding example corresponds t o Calatayud historical city, where subsidence severely damages highly valua ble monuments. The subsidence resulting from the underground karstification of evaporites has determined or influenced the generation of some importan t modem lacustrine basins like Gallocanta, Fuente de Piedra and Banyoles la kes. The sudden formation of sinkholes due to the collapse of cave roofs is relatively frequent in some evaporite outcrops. Very harmful and spectacul ar subsidence activity is currently occurring in the Cardona salt diapir wh ere subsidence has been dramatically exacerbated by mining practices.