The geochemical and isotopic record of evaporite recycling in spas and salterns of the Basque Cantabrian basin, Spain |
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| Authors: | V Iribar B Ábalos |
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| Institution: | Departamento de Geodinámica, Universidad del País Vasco, PO Box 644, E-48080 Bilbao, Spain |
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| Abstract: | Evaporite outcrops are rare in the Basque Cantabrian basin due to a rainy climate, but saline springs with total dissolved solids ranging from 0.8 to 260 g/L are common and have long been used to supply spas and salterns. New and existing hydrochemistry of saline springs are used to provide additional insight on the origin and underground extent of their poorly known source evaporites. Saline water hydrochemistry is related to dissolution of halite and gypsum from two evaporitic successions (Triassic “Keuper” and Lower Cretaceous “Wealden”), as supported by rock samples from outcrops and oil exploration drill cuttings. The δ34S value of gypsum in the Keuper evaporites and sulfate in the springs is δ34SSO4 = 14.06 ± 1.07‰ and δ18OSO4 = 13.41 ± 1.44‰, and the relationship between Cl/Br ratio of halite and water shows that waters have dissolved halite with Br content between 124 and 288 ppm. The δ34S value of gypsum in the Wealden evaporites and sulfate in the springs is δ34SSO4 = 19.66 ± 1.76‰, δ18OSO4 = 14.93 ± 2.35‰, and the relationship between Cl/Br ratio of halite and water shows that waters have dissolved halite with Br content between 15 and 160 ppm. Wealden evaporites formed in a continental setting after the dissolution of Keuper salt. Gypsum δ34SSO4 and δ18OSO4 modification from Keuper to Wealden evaporites was due mainly to bacterial SO4 reduction in an anoxic, organic matter-rich environment. Saline springs with Wealden δ34SSO4 values are present in a 70 × 20 km wide area. Saline water temperatures, their δ2HH2O and δ18OH2O values, and the geological structure defines a hydrogeological model, where meteoric water recharges at heights up to 620 m above spring levels and circulates down to 720 m below them, thereby constraining the height range of evaporite dissolution. Groundwater flow towards saline springs is driven by gravity and buoyancy forces constrained by a thrust and fault network. |
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