Quaternary regional evolution based on karst cave geomorphology in Picos de Europa (Atlantic Margin of the Iberian Peninsula)

Abstract

Alpine caves have attracted considerable geomorphological, paleoenvironmental and hydrogeological interest since climate, glaciations, relief uplift, fluvial incision and karst aquifer control their evolution. In the Atlantic Margin of the Iberian Peninsula, Picos de Europa mountains is among of the most important karst areas of the World containing some of the deepest caves explored today. In addition, these mountains represent a reference site for the study of the Last Glacial Cycle in the SW of Europe. This work aims to reconstruct the Pliocene-Quaternary evolution of this region based on geomorphological and geochronological research (U/Th and Al/Be) carried out in four alpine caves. Cave geomorphological mapping evidences that 12 km of studied caves are made up of 47% vadose canyons and shafts, 45% phreatic/epiphreatic conduits organised in six cave levels, and 7% breakdown-modified passages. Their deposits are characterized by speleothems, including flowstones, that represent ancient cave pavements, fluvial terrace deposits with allochthonous clasts, slackwater deposits related to cave floods, and debris deposits produced by breakdown. One 26Al/10Be burial age indicates a minimum age of 2.1 ± 0.5 Ma for the caves origin, allowing estimation of the mountain uplift at 0.15–0.25 mm·a−1 since the Late Pliocene. Twenty-eight new 234U/230Th ages and another six previous speleothem ages give ages ranging from MIS 8 to 1. The speleogenetic model comprises six phases of regional evolution. Phase 1: main development of cave levels with SE-directed phreatic flow in the NW of Picos de Europa, in a karst partially or totally covered by the detrital Permian-Mesozoic cover, presently eroded. Phase 2: erosion of the Permian-Mesozoic cover and onset of vadose conditions before 260 ka, in a karst-affected by fluvial captures. Phase 3: cave infill during 220–145 ka, probably caused by the erosion of Stephanian detrital outcrops. Phase 4: erosion of cave infill during 125–45 ka. Phase 5: apparent pause in the speleothem formation during 45–25 ka related to dry and cold regional conditions. Phase 6: reactivation of the speleothem precipitation since 25 ka. Regional climate, fluvial incision and the ancient presence of detrital outcrops at the surface appear to have been the main factors that controlled the cave evolution and regional geomorphological evolution throughout Pliocene and Quaternary times.