Fibrous clay minerals as lithostratigraphic markers in a Cenozoic continental basin (Sado Basin, Portugal)

• Palaeogene deposits show a high amount of palygorskite.
• Illite is pointed out as the most probable precursor of palygorskite.
• Clay mineral associations were used for diagenetic analysis.
• Palygorskite was used as lithostratigraphic marker for the deposits studied.

The Sado Basin is located in the southwestern part of the Iberian Massif. In order to improve the lithostratigraphic and palaeoenvironmental knowledge of this Cenozoic basin, mineralogical and geochemical profiles were assessed on three lithostratigraphic units: Ervidel and Vale do Guizo (Eocene) formations and Esbarrondadoiro (Miocene) Formation.Samples representative of the most exposed outcrops of these units were collected and analyzed by XRD, XRF and SEM–EDAX. Mineralogical and chemical results were analyzed statistically using PERMANOVA, principal components analysis and factor analysis. SEM–EDAX was used to calculate the chemical formula of palygorskite. Clay mineral associations were used as stratigraphic markers as well as for palaeoenvironmental reconstructions and diagenetic analyses.