Geochemistry and Mineralogy of Clays from the Algarve Basin, Portugal: A Multivariate Approach to Palaeoenvironmental Investigations

Lower Cretaceous and Triassic-Hettangian clays from the Algarve Basin ( South Portugal) were deposited in distinct environments during basin subsidence due to Pangea break up. The mineralogy, geochemistry and granulometry of 45 samples of both clay groups were studied, using X-ray diffraction, instrumental neutron activation analysis, X-ray fluorescence and laser diffractometry by attenuation of X-rays. Multivariate statistical techniques ( principal component analysis and K-means clustering), after scandium normalization of chemical contents, were used to study simultaneously all data, enabling to distinguish the different clay types by establishing the best discriminatory parameters: high contents ( in decreased order) of kaolinite, quartz, Chemical Index of Alteration ( CIA), Ti, goethite, Hf, Si, > 63 mu m fraction and Zr, characterize the group of clays from Cretaceous; while high contents of hematite, Mg, mobile elements (K, Cs and P), illite, Ca, phyllosilicates in general, Mn and dolomite, are associated to Triassic-Hettangian clays. This approach revealed useful to facilitate the integration of all data, as was possible to confront the geochemistry with the mineralogy, enabling the best palaeoenvironmental interpretations. In fact, the clay mineral assemblage is significantly influenced by the dominant weathering process and provides information on changes in aridity/humidity patterns.