Abstract

This work focusses on the role of the depositional environment on clay diagenesis in a sandstone-shale system. Five sections located in the southern flank 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). Facies associations comprise 0.5 to 3m coarsening upward sequences of interbedded shales and siltstones with hummocky cross stratified sandstones, or structureless sandstones. The clay-mineral assemblages are mainly composed of illite and illite-smectite mixed-layers for the Supra-Urgonian complex (Upper Albian-Lower Cenomanian) and of illite and chlorite for the Wealden complex (Valanginian-Barremian). The diagenetic processes that led to these clay mineral assemblages involved mainly the transformation of smectite and I/S mixed layers to illite and the precipitation of Fe-rich chlorite in the pore spaces of Wealden sandstones. All data suggest that chlorite formation was controlled primarily by the initial mineralogy, initial chemistry, grain size and vertical facies organisation. Burial diagenesis took place in a closed-system with no elemental exchange by fluids. Dolomite and ankerite dissolution and the breakdown of Fe-oxides released of Fe and Mg allowing the precipitation of Fe-rich chlorite especially in coarser grained sandstones located near the top of coarsening upward sequences.

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