Diagenesis, palaeoclimate and tectono-sedimentary influences on clay mineralogy and stable isotopes from Upper Cretaceous marine successions of the Basque-Cantabrian Basin (N Spain)

Clay mineralogy and whole-rock stable isotopes (delta O-18 and delta C-13) of Upper Cretaceous marly sediments on the Basque-Cantabrian Basin have been integrated to determine the main effects of diagenesis, palaeoclimate and tectono-sedimentary factors in sections belonging to deep- (Barrika) and platform-marine (Isla de Castro, Villamartin and Olazagutia) settings. The mean values for the clay assemblages and delta O-18 exhibit notable differences among the sections, partially explainable by the influence of diagenesis. The Barrika sediments, with more diagenetically advanced illite-smectite (I-S) mixed-layer (R1, 70% illite), authigenic chlorite, and low delta O-18 (-4.05 parts per thousand. PDB), experienced higher diagenetic grade than Isla de Castro and Olazagutia, which have R0 I-S (20% illite) and heavier delta O-18. Villamartin was also affected by higher diagenesis than Isla de Castro and Olazagutia, given the occurrence of R1 I-S (60% illite) and low delta O-18 (-4.11 parts per thousand. PDB). However, the absence of other clays in Villamartin (e.g. authigenic chlorite) is indicative of less diagenetic grade than Barrika. These results show the useful integration of clay mineralogy and stable isotopes to detect different diagenetic, grades in distinct marine successions of the same basin. Despite being influenced by diagenesis, the clay mineralogy partially preserves its inherited signature. This allows detection of major contents of I-S and mica, and minor kaolinite, interpreted as indicative of warm palaeoclimatic conditions. High kaolinite content in Villamartin and absence of kaolinite in Isla de Castro, though, are considered to be a product of neither diagenesis nor palaeoclimatic influences. Instead, tectono-sedimentary causes, related to unsuitable conditions for clay formation and transport from the local source areas, contributed to original clay differences. The inferred effects of diagenesis, palaeoclimate and tectono-sedimentary factors make this work important to show the potentially great variety of controls on the clay mineralogy of marine sections, which are often uncritically treated in studies concerning the Late Cretaceous. (C) 2007 Elsevier Ltd. All rights reserved.