Estudio diagenético en conchas de braquiópodos de la cuenca del Guadiato ( Mississippiense, SO del Macizo Ibérico)

Resumo

Detailed petrographic and geochemical analyses of brachiopod shells from the Guadiato Carboniferous basin (Córdoba, SW Iberian Massif) has been carried out in order to evaluate their textural and geochemical preservation degree and their potential to be used as indicators of paleoenvironmental changes during the Carboniferous. The preservation degree of the shell´s microstructure has been evaluated by petrographic microscope, cathodoluminescence (CL) and scanning electron microscope (SEM). Under CL, ventral valves are mainly nonluminescent (NL) or nonluminescent with small slightly luminescent areas (NL-SL). In contrast, most of the dorsal valves are luminescent with moderate luminescent areas (ML-L). Under SEM, the samples show very well preserved lamellar and fibrous microstructures. The elemental contents (Ca, Mg, Sr, Mn and Fe) for the NL portions agree with elemental contents of modern brachiopods in equilibrium with ambient seawater and that have not undergone significant diagenetic modification. Therefore, our findings reveal a better microstructural preservation of the ventral valves of Gigantoproductids than the dorsal valves. The NL and NL-SL shell portions present the better microstructural and geochemical preservation and are interpreted to reflect the original isotopic and chemical composition of the marine water from which they precipitated.