A mineralogical and geochemical study of the Gabasa outcrop (Huesca, NE Spain) was undertaken. It consists of Early Oligocene marly and clayey fluvial and lacustrine (playa-lake) sediments. The phases detected were quartz, amorphous silica, calcite, dolomite, palygorskite, illite, interstratified illite-smectite, Al-smectite and Mg-smectite. The palygorskite expands with ethyleneglycol. Statistical analysis of the geochemical data shows that the rare earth elements and transition trace elements are basically associated with the detrital phyllosilicates, although a considerable amount of the latter is contained in the palygorskite (ΣREE = 60–70 ppm, Cr+Co+Ni+V+Zn+Cu = 120–150 ppm), in contrast to the normally low values for neoformed minerals. This fact, together with the significant presence of Al and Fe in the palygorskite, suggest genesis involving alteration by dissolution of the 2:1 structure of the illite and/or Al-smectite, followed by re-ordering in a fibrous structure.