Sinkholes and caves related to evaporite dissolution in a stratigraphically and structurally complex setting, Fluvia Valley, eastern Spanish Pyrenees. Geological, geomorphological and environmental implications

Abstract

Evaporite karst and sinkhole development is analysed in a geologically complex area of NE Spain, including four evaporite units with different characteristics and affected by compressional and extensional tectonic structures. The exposed paleosinkholes, including remarkable Early Pleistocene paleontological sites, provide valuable information on the subsidence mechanisms and reveal the significant role played by interstratal karstification in the area. These gravitational deformation structures, including hectometre-scale bending folds and oversteepened normal faults, strongly suggest that the present-day compressional regime inferred in previous studies may be largely based on the analysis of non-tectonic structures. Two gypsum caves ca. 1km long show that passages with restricted cross-sectional area may produce large breccia pipes and sinkholes thanks to the removal of breakdown boulders by high-competence episodic floods. Moreover, the upward progression of cave ceilings by paragenesis and condensation dissolution contributes to increase the probability of sinkhole occurrence. An inventory of 135 sinkholes together with their geological and geomorphological context has been developed. This data base has been used to infer several properties of the sinkholes with practical implications: a magnitude and frequency scaling relationship, spatial distribution patterns, dominant controlling factors and risk implications.