Diagenetic alteration in monopleurid rudist shells from the Urgonian Complex of El Castillo Cape (Lower Albian, Gorliz, Bizkaia)

The rudist assemblages of the Urgonian limestones of El Castillo Cape (lower Albian) consist largely on bouquets and thickets of caprotinids in whose interspaces grew bouquets of monopleurids, all deposited in a shallow marine platform. The complex diagenetic history suffered by El Castillo Cape sediments, from early stages (vadose meteoric dissolution) to more advanced ones (neomorphism), entrained the monopleurid shell geochemistry. Detailed chemical analysis along the intergrowth texture of two monopleurid shells show the different luminescent and geochemical behaviour of each part of the shells in response to diagenetic processes. The analyzed specimens are thought to have suffered a medium diagenetic degree, in spite of the microstructures are moderately preserved. So it can be deduced from the Sr-Na low mmol/mol values, the Mn-Fe high mmol/mol values and the homogeneous distribution of luminescence following the shell microstructures. The hipostracum and inner miostracum are thought to have been the more accessible parts of the shell for the influence of diagenetic fluids, and show a clear evidence of neomorphism. Only a part of the miostracum of one of the specimens still partially retains the original composition, displaying «saw-tooth» cationic profiles and behaving non-luminescen