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

Zusammenfassung

The radiolitid shells included in the Urdax limestones (middle Aptian-lower Cenomanian of northern Navarra) have been studied for evaluation of diagenetic alteration and degree of preservation of the original biological composition. A first draft of diagenetic sequence has been traced, from early �dogtooth� calcite cements to further fracturing to later blocky calcite cement postdating fracturation. In spite of their typical �honeycomb� microstructure, radiolitd shells may have suffered from a lighter diagenetic alteration in comparison to caprinids, because their shell is not recrystallized and appears non-luminescent. In addition, the cationic ratios, particularly Sr+2/Ca+2 and Na+2/Ca+2, demonstrate a lesser degree of diagenetic alteration in non-fractured shell zones (ZNF) when compared to fractured ones (ZF). The Sr+2/Mg+2 values of ZF are closer to the abiotic calcite line than those of the ZNF. The Sr+2/Mn+2 values of the ZF do not lie into the geochemical field of extant bivalve shells, whereas a 40% of the same values in the ZNF are included in such a field. A further geochemical comparison with the results obtained for caprinid shells suggest that radiolitids could preserve their shell pristine composition in a higher degree than caprinids.