Origin and evolution of tufa mounds related to artesian karstic springs in Isona area (Pyrenees, NE Spain)

Abstract

This work analyses a suite of relict tufa mounds generated by artesian karstic springs in Isona area (Spanish Pyrenees). Geological and geophysical data (seismics and vertical electrical resistivity soundings) indicate that the location of the discharge area in which the spring mounds formed was controlled by (1) a bulge in the axial zone of the anticline that affects the Areny-Montsec aquifer, with the consequent thinning of the overlying confining unit and (2) N-S and E-W trending extensional faults. These uncommon meteogene mounds occur in two stepped morphostratigraphic units that constitute the caprock of a mesa 9 km2 in area. The upper tufa complex is 47 m thick and has yielded several U/Th dates of >350 ka. The obtained U/Th ages for the mounds of lower tufa complex, 10 m thick, range from >350 to 214 ± 11 ka. The sedimentological analysis of the scarce exposures and electrical resistivity profiles show the same morphological and depositional components as those described in geothermal springs: (1) Cylindrical vents; (2) Pools fed by the vents and dammed by annular tufa barriers; (3) Tufa barriers (rimstones) constructed by overflowing waters through vertical accretion and progradation. These rimstones may have overhanging upflow sides. (4) Slope tufa facies with terracettes and microgours.