DIAGENESIS OF THE CENTRAL BASQUE-CANTABRIAN BASIN (IBERIAN PENINSULA) BASED ON ILLITE-SMECTITE DISTRIBUTION

X-ray diffraction was used to analyse the distribution of clay minerals in the < 2-mu-m fraction of the lutitic and marly facies from the centre of the Basque-Cantabrain Basin (Basque Arc), where the sedimentary section is 2000 to 10,000 m thick. Most of the deposits were laid down during the Cretaceous and Paleogene and were related to the opening and closing of the Bay of Biscay. The most noteworthy variations are in kaolinite, smectite and mixed-layered (R = 0, R = 1 and R greater-than-or-equal-to 3) illite-smectite, which can be ascribed both to provenance and to diagenesis. A general diminution in expandability is related to the gradual transformation of smectite to illite from south to north, and with depth. Temperature, residence time and chemical activity during diagenesis are the factors that had greatest influence on the changes in the original mineral assemblages inherited from the source area. On the basis of the clay minerals and the Kubler index, a general scheme is proposed for the diagenetic evolution of the area from the initial stages to anchimetamorphism.