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Bureau Seminar, November 13, 2009

Evaporite Karst in Northern Spain

Bob Loucks
Bureau of Economic Geology

Evaporite karst is a dominant geomorphic feature in northern Spain. A thick section of near-surface and exposed evaporites, ranging in age from Triassic to Holocene, has undergone dissolution starting in the Tertiary and continuing today. Evaporite dissolution results in the development of sinkholes, formation of lakes, ponds, and salinas, and thickened alluvial and fluvial deposits. The evaporite karst is being intensively studied because of the geohazards associated with subsidence and sinkholes, especially in areas of municipalities and along the "fast-train" rail line. These excellent exposures of evaporite karst have additional research value as analogs for evaporite paleokarst, which is not as well understood as carbonate paleokarst.

Because of the excellent, widespread exposures of evaporites in northern Spain, it may be considered the type area for Tertiary to Recent evaporite karst processes. In general there are three stratigraphic settings for evaporite karst (Gutiérrez et al., 2001). These are interstratal, mantled, and uncovered evaporite karst. Interstratal karst forms where dissolution of evaporites occurs between lithified strata such as carbonates. This type of evaporite karst can form caverns and collapsed zones that contain both transported and collapsed breccias. These zones can cover many square miles. In some cases, the residue zone may be impervious and form an intrastratal seal. Above these zones, suprastratal deformation may occur as folds, sags, and faults. Mantled karst occurs where unlithified sediments are deposited above the evaporite. As the evaporite dissolves, caverns form in the evaporite. When these caverns collapse, a sinkhole will form at the surface through the overlying sediments. These sinkholes can occur catastrophically or develop by numerous stages of small collapsing events. Mantled karst produces most of the geohazards encountered in northern Spain. Surface karst occurs where evaporites are exposed, and caves and saline lakes commonly form at the surface. Results from investigating this evaporite karsted area can be applied to such producing formations as the Devonian Grosmont, Mississippian Madison, Carboniferous Lisburne, and possibility the Lower Ordovician Ellenburger.


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