Clay mineral assemblages in flysch from the Campo de Gibraltar area (Spain)

In order to determine the relative influence of palaeoenvironmental and dia genetic processes in clay assemblages, as well as their significance, both fine- and coarse-grained sediments from the Campo de Gibraltar flysch have been studied by means of X-ray diffraction, optical and electron microscopy , and chemical analysis. Diagenetic modifications appear to be lithological ly controlled and mainly affect coarse-grained sediments, where Fe-chlorite s, illite and kaolinite are the more characteristic authigenic clay mineral s. The evolution of detrital assemblages, determined in fine-grained beds, indicates that, from Cretaceous to Eocene times, clay mineralogy, character ized by the opposite kaolinile+smectite and illite + I-S mixed-layer assemb lages, was mainly controlled by sources, climate and transport processes. O n the other hand, from the Oligocene, clay mineral assemblages, characteriz ed either by the abundance of kaolinite, or by the illite+chlorite associat ion, mainly reflect the petrology of source rocks, as a consequence of clim atic cooling and the increasing tectonic activity, which impede the develop ment of soils.