Role of the depositional environment on clay diagenesis: example of the Lower Cretaceous of the Basco-Cantabrian Basin (northern Spain)

This work focusses on the role of the depositional environment on clay diag enesis in a sandstone-shale system. Five sections located in the southern f lank of the Bilbao anticline of the Basco-Cantabrian Basin (northern Spain) have been studied. Each section reveals sedimentary sequences accumulated under similar depositional conditions (prograding deltaic environment). Fac ies associations comprise 0.5 to 3m coarsening upward sequences of interbed ded shares and siltstones with hummocky cross stratified sandstones, or str uctureless sandstones. The clay-mineral assemblages are mainly composed of illite and illite-smectite mixed-layers for the Supra-Urgonian complex (Upp er Albian-Lower Cenomanian) and of illite and chlorite for the Wealden comp lex (Valanginian-Barremian). The diagenetic processes that led to these cla y mineral assemblages involved mainly the transformation of smectite and I/ S mixed layers to illite and the precipitation of Fe-rich chlorite in the p ore spaces of Wealden sandstones. All data suggest that chlorite formation was controlled primarily by the initial mineralogy, initial chemistry, grai n size and vertical facies organisation. Burial diagenesis took place in a closed-system with no elemental exchange by fluids. Dolomite and ankerite d issolution and the breakdown of Fe-oxides released of Fe and Mg allowing th e precipitation of Fe-rich chlorite especially in coarser grained sandstone s located near the top of coarsening upward sequences.