Diagenetic aspects of Orbiculoidea in phosphatic concretions (Devonian, unit 2, Chapada group, MS): taphonomic and paleoenvironmental implications

Abstract

The mineralogical characteristics of tegument can influence differential preservation processes among organisms (e.g. dissolution and concretization). In this context, stand out the chitin-phosphatic shells of orbiculoid brachiopods, fossil group that also composed the Malvinokaffric Relm. These fossils are usually preserved in phosphatic concretions in Devonian sediments of the Paraná Basin (Alto Garças Sub-basin). It is possible that the phosphatic nature of these shells has propitiated favorable microenvironment for the concretization, since the calcium phosphate can elevate the formation of apatite concretions. Hence, this group can be used as a model in studies of diagenesis concerned with alteration processes such as dissolution and deposition of calcium phosphate in concretions. In addition, the taphonomic biases involved in the process, may have taxonomical implications. Thus, the objectives of this study are: (1) to investigate micro and ultrastructure changes in orbiculoides preserved in concretions and other types of fossilization; (2) to understand diagenetic aspects in these phosphatic concretions and; (3) to investigate the relations between the presence of Rare Earth Elements (REE) and the formation of concretions in orbiculoides. Therefore, specimens collected near Rio Verde (Alto Garças Sub-Basin, MS) will be processed for observations under stereomicroscopy, MEV and petrography as well as other paleometric techniques, such as Raman spectroscopy, X-ray fluorescence, EMPA, EDXRF to assist in the investigation of geochemical aspects of the fossililization of those specimens. (AU)