The Quaternary activity of the Estella diapir from the uplift record of fluvial terraces, pediments and cave sediments in the Western Pyrenees, Spain

Abstract

This work analyses the Quaternary landforms, geomorphic changes and stratigraphic evidences of evaporite dissolution and halokinesis in the Estella salt diapir (Basque-Cantabrian Basin). The mapping of Quaternary deposits demonstrates that karstic subsidence and diapirism happen at the same time. The diapir geometry is characterised by a prominent 4.5 km-long fault scarp attributed to the uplift of the diapir and a fluvial gypsum escarpment along the northern and eastern diapir edges. The dissolution of the gypsiferous caprock is responsible for the development of a large number of collapse sinkholes, the local thickening of terrace deposits and a network of endokarstic conduits connected to saline springs that cause a notable increase in the salinity of the Ega river across the diapir (70% electrical conductivity increase). The 40 m vertical displacement of T4 terrace and P5 pediment deposits, the anomalous longitudinal profile of the old terraces, the deflection of infilled-valleys at the base of the fault scarp, the occurrence of wind gaps and the continuous lateral migration of the Ega river away from the diapir support salt upwelling since 644 ± 81 ka ago at a long-term, average uplift rate of 0.05–0.07 mm/yr. The 12 m offset of T8 terrace, the steepening of the Ega river floodplain and the tilting of the upper level and the antigravitative erosion genesis of the Longinos cave point to a faster diapir rising rate of 0.23–0.57 mm/yr in the last 36 ± 15 ka coinciding with a period of increasing river incision. The lack of tectonic activity in the region suggests that diapirism is enhanced by erosional unloading related to fluvial entrenchment. The thickening of pediment P5 supports the north-northeast flow of salt towards the river valley.