Multiepisodic evaporite sedimentation as an indicator of palaeogeographical evolution in foreland basins (South-eastern Pyrenean basin, Early–Middle Eocene)

Extensive deposition of marine evaporites occurred during the Early–Middle Eocene in the South-eastern Pyrenean basin (north-east Spain). This study integrates stratigraphic and geochemical analyses of subsurface data (oil wells, seismic profiles and gravity data) together with field surveys to characterize this sedimentation in the foredeep and adjacent platform. Four major evaporite units were identified. The oldest was the Serrat Evaporites unit, with a platform-slope-basin configuration. Thick salina and sabkha sulphates accumulated on the platform, whereas resedimented and gravity-derived sulphates were deposited on the slope, and salt and sulphates were deposited in the deep basin. In the subsequent unit (Vallfogona evaporites), thin sulphates formed on the platform, whereas very thick siliciclastic turbidites accumulated in the foredeep. However, some clastic gypsum coming from the platform (gypsarenites and gypsum olistoliths) was intercalated in these turbidites. The following unit, the Beuda Gypsum Formation developed in a sulphate platform-basin configuration, where the topography of the depositional surface had become smooth. The youngest unit, the Besalu Gypsum, formed in a shallow setting. This small unit provides the last evidence of marine influence in a residual basin. Sulphur and oxygen isotope compositions are consistent with a marine origin for all evaporites. However, δ34S and δ18O values also suggest that, except for the oldest unit (Serrat Evaporites), there was some sulphate recycling from the older into the younger units. The South-eastern Pyrenean basin constitutes a fine example of a foreland basin that underwent multiepisodic evaporitic sedimentation. In the basin, depositional factors evolved with time under a structural control. Decreasing complexity is observed in the lithofacies, as well as in the depositional models, together with a diminishing thickness of the evaporite units.