CLAY MINERAL VARIATIONS ASSOCIATED WITH DIAGENESIS AND LOW-GRADE METAMORPHISM OF EARLY CRETACEOUS SEDIMENTS IN THE CAMEROS BASIN, SPAIN

The clay mineral distribution in the Early Cretaceous depositional sequences along the Cameros Basin has been established on the basis of XRD traces and TEM/EDAX analyses. Samples from the Latest Berriasian-Barremian depositional sequences are characteristic of epimetamorphic conditions. Illite 'crystallinities' are broader than expected (0.35-4.490 degrees Delta 2 theta), due to the consistent presence of K-mica-paragonite mixed-layer and discrete paragonite associated with the illite 10 Angstrom peak. The Late Barremian-Early Aptian depositional sequence, around the depocentral sector of the basin, represents a sudden change to anchimetamorphic conditions, marked by the presence of pyrophyllite and rectorite and by a significant decrease in the Al-IV content of the chlorites. Towards the northwestern border of the basin, this sequence was affected by deep diagenetic conditions, as deduced from the clay mineral association and the 'crystallinity' values (0.57 degrees Delta 2 theta). The changes in the clay mineral assemblages and 'crystallinity' data can hardly be explained in terms of the varying burial depth and are related to changes in the circulation of fluids associated with varying facies (modal composition, grain size). Permeability is regarded as the main factor controlling the circulation of these migratory fluids.