Alteración diagenética diferencial en conchas de rudistas (Hippuritacea, Bivalvia) del Cretácico Medio de Ereño (Bizkaia, País Vasco).

Resumen

Two geochemical transects have been performed along two sections of rudist shells (one requienid and one caprotinid) from the Middle Cretaceous of Ereño (Basque Country). Diagenetic alteration in both shells was different, as it can be deduced from geochemical analysis using electronic microprobe. The requienid shell still retains some original chemical composition, as inferred from the relatively high-Sr2+ content in the inner part of the shell (miostracum), to diminish towards the skeletal extremes (hipostracum and periostracum). A contrary trend in typical «diagenetic» cations such as Fe2+ and Mn2+ is observed. Interestingly, the Mg2+ -content in the requienid shell is very low in the miostracum to increase in hipostracum and miostracum, which suggests a dolomitizative effect. On the other hand, the caprotinid shell shows a different geochemical behaviour, because Sr2+ is depleted when compared to its content in the requienid shell, and the relative chemical homogeneization in the rest of cations (Ng+, Mg2+, Fe2+ and Mn2+) is also evident. From these data, we move to affirm that the caprotinid shell is thought to have been more intensely affected by diagenesis than the requienid one. We suggest the possibility that tectonic stress rather than the primary geochemical composition of the shells was the main factor that controlled the diagenetic degree suffered by the rudist shells.