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 seq uences along the Cameros Basin has been established on the basis of XR D traces and TEM/EDAX analyses. Samples from the Latest Berriasian-Bar remian depositional sequences are characteristic of epimetamorphic con ditions. Illite 'crystallinities' are broader than expected (0.35-4.49 0 degrees Delta 2 theta), due to the consistent presence of K-mica-par agonite mixed-layer and discrete paragonite associated with the illite 10 Angstrom peak. The Late Barremian-Early Aptian depositional sequen ce, around the depocentral sector of the basin, represents a sudden ch ange to anchimetamorphic conditions, marked by the presence of pyrophy llite 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 t he clay mineral association and the 'crystallinity' values (0.57 degre es Delta 2 theta). The changes in the clay mineral assemblages and 'cr ystallinity' data can hardly be explained in terms of the varying buri al depth and are related to changes in the circulation of fluids assoc iated with varying facies (modal composition, grain size). Permeabilit y is regarded as the main factor controlling the circulation of these migratory fluids.