Alteración diagenética en conchas de rudistas caprínidos del Cretácico Medio de Urdax (Navarra)

The caprinid rudist shells of the middle Aptian-lower Cenomanian limestones of Urdax (Navarra) have been studied for evaluation of the diagenetic degree that suffered from. Strongly recrystallized, the shells only preserve the pallial canals and show sparry calcite of different crystal sizes depending on hipostracum or miostracum layers. A later recrystallization produced a clear increase of Mn+2 and Fe+2 content in nondisplacive veins filled by coarse undulose calcite. The very low Sr+2 /Ca+2 and Na+ /Ca+2 values suggest a high diagenetic degree, although their geochemical profiles in «saw-teeth» still remember to us about a certain vital effect. The Fe+2/Ca+2 and Mn+2 /Ca+2 values sharply increase in late-recrystallized zones and veins having strong yellow CL colors. Finally, the Sr+2/Mg+2 values are close to those of abiotic calcite, and the Sr+2/Mn+2 values do not coincide with those for extant bivalve shells. In all case, the fact that the original microstructure and composition of caprinid shells could make them more particularly propitious for diagenetic alteration than specimens of other families like radiolitids should not be discarded