Early diagenetic modification of the inoceramid shells (Bivalvia). Lower Maastrichtian. Basque-Cantabrian Basin

The petrographic analysis of the inoceramid shells allows to identify some microstructures, which suggest their early modification, previously and during the first burial stages. In this sense, it is important to emphasize the borings produced by epibionts and the formation of an early ferrous-manganese coating in the outer surface of the shell. The desegregation of the prismatic framework and its dispersion in the sediment are other processes that took place during the early stages of burial. The study of the post-mortem modification of the shells brings forward some evidences about the marine bottom oxygenation in the Basque-Cantabrian Basin, just before the inoceramid extinction near the lower-upper Maastrichtian boundary (~69,3 m.a.