Belemnite taphonomy (Upper Jurassic, Western Tethys) part II: Fossil–diagenetic analysis including combined petrographic and geochemical techniques

Abstract

Fossil–diagenetic features were analyzed on 56 belemnite rostra from the Pozo Cañada section (External Prebetic), as well as 31 belemnite rostra from the Río Segura (Internal Prebetic), both from the Upper Oxfordian–Lower Kimmeridgian. They mainly correspond to Hibolithes and, secondarily to Belemnopsis. Fossil–diagenetic processes were analyzed in each specimen, using petrographic (conventional, cathodoluminescence, epifluorescence and scanning electron microscopy) and geochemical (elemental and stable isotopes) techniques. The most common fossil–diagenetic processes are dissolution, calcite cementation, and recrystallization of the apical zone and outer growth rings of belemnite rostra. These processes may appear enhanced by fracturing and stylolite formation. Petrographic study also reveals that the alternation of cloudy and clear concentric growth areas displayed by many belemnite rostra corresponds to an early diagenetic feature in origin. However, an original concentric growth pattern is also observed under epifluorescent microscopy and BSEM. This growth pattern fits with changes in the Mg and S content of the rostra. Although fossil–diagenetic processes typically make specimens non-suitable for paleoenvironmental interpretations, microsampling of petrographically altered and non-altered areas from the same specimens, performed directly from thin sections after petrographic study, allowed us to obtain excellent geochemical results suitable for paleoenvironmental interpretations. These geochemical analyses moreover demonstrates that caution should be taken if elemental analyses are used as the most significant criteria for discriminating diagenetically altered and non-altered belemnite samples.