Cyclostratigraphic analysis of the Middle to lower Upper OrdovicianPostolonnec Formation in the Armorican Massif (France): integratingpXRF, gammay-ray and lithological data

International audienceThe Middle to lower Upper Ordovician sections of the Crozon Peninsula area (Postolonnec Formation, ArmoricanMassif, western France) show multi-order eustatic sea-level changes (Dabard et al., 2015). The sections arecharacterized by siliciclastic facies, which were deposited in tidal to storm-dominated shelf environments. Dabardet al. (2015) analysed the facies, their stacking patterns, and gamma-ray data and applied backstripping toidentify subsidence and several orders of sea-level change. The main stratigraphic constraints are coming from(chitinozoan) biostratigraphy. The 3th to 5th orders changes are hypothesized to correspond to various frequenciesrelated to astronomical forcing.This study investigates the potential added value of portable X-Ray Fluorescence (pXRF) and the applicationof spectral analyses. High-resolution (cm-scale) non-destructive pXRF and natural gamma-ray measurementswere carried out on 14 m of section that was equally logged on a cm resolution. The pXRF measurementson the surface of the outcrops are compared with earlier results of wavelength dispersive XRF spectrometryand ICP-MS. The potassium records of the pXRF and gamma-ray logs are comparable and essentially reflectlithological variations (i.e. between mudstone and coarse sandstones). Other reliably measured elements alsoreflected lithological aspects such as clay-sandstone alternations (e.g. K, Rb, Ti), placer locations (Zr, Ce, Ti) andpotentially clay mineralogy and condensation horizons (Ni, Zn, Co, Mn).Spectral analyses of the various proxies (lithology, natural gamma-ray and pXRF) are compared with eachother. Both the new high-resolution data (14 m of section) as well as the published low-resolution data (whichspan almost 400 m of Darriwilian-Sandbian) were analyzed. The study reveals strong indications for the imprintof obliquity, precession and eccentricity. Obtaining age constraints, in addition to the existing biostratigraphicalframework is a challenge in these sections, but would help to resolve temporal uncertainties and confirm ourinterpretations. The relative strength of the potential obliquity and precession-eccentricity signals also can providefurther insights in the global glaciation history of the Middle to Late Ordovician given that a larger obliquitycomponent can be expected if there was a more developed polar ice sheet on the Gondwanan palaeocontinent.Dabard M.P., Loi A., Paris, F., Ghienne J.F., Pistis M., and Vidal M. (2015): Sea-level curve for the Middleto early Late Ordovician in the Armorican Massif (western France): Icehouse third-order glacio-eustaticcycles. Palaeogeography, Palaeoclimatology, Palaeocology, 436, 96-111, doi:10.106/j.palaeo.2015.06.03