Cyclostratigraphy of the Middle to Upper Ordovician successions of the Armorican Massif (western France) using portable X-ray fluorescence

Abstract. To expand traditional cyclostratigraphic numerical methods beyond their common technical limitations and apply them to truly deep-time archives, we need to reflect on the development of new approaches to sedimentary archives that are not traditionally targeted for cyclostratigraphic analysis but that frequently occur in the impoverished deep-time record. Siliciclastic storm-dominated shelf environments are a good example of such records. Our case study focuses on the Middle to Upper Ordovician siliciclastic successions of the Armorican Massif (western France) that are well-studied examples in terms of sedimentology and sequence stratigraphy. In addition, these sections are protected geological heritage due to the extraordinary quality of the outcrops. We therefore tested the performance of non-destructive high-resolution (centimeter-scale) portable X-ray fluorescence and natural gamma-ray analyses on an outcrop to obtain major and trace element compositions. Despite the challenging outcrop conditions in the tidal beach zone, our geochemical analyses provide useful information regarding general lithology and several specific sedimentary features such as the detection of paleo-placers or the discrimination between different types of diagenetic concretions such as nodules. Secondly, these new high-resolution data are used to experiment with the application of commonly used numerical cyclostratigraphic techniques on this siliciclastic storm-dominated shelf environment, a non-traditional sedimentological setting for cyclostratigraphic analysis. In the parts of the section with a relatively homogeneous lithology, spectral power analyses and bandpass filtering hint towards a potential astronomical imprint of some sedimentary cycles, but this needs further confirmation in the absence of more robust independent age constraints.