Cyclostratigraphic analysis of the Middle to lower Upper Ordovician Postolonnec Formation in the Armorican Massif (France): integrating pXRF, gammay-ray and lithological data
Résumé
The Middle to lower Upper Ordovician sections of the Crozon Peninsula area (Postolonnec Formation, Armorican
Massif, western France) show multi-order eustatic sea-level changes (Dabard et al., 2015). The sections are
characterized by siliciclastic facies, which were deposited in tidal to storm-dominated shelf environments. Dabard
et al. (2015) analysed the facies, their stacking patterns, and gamma-ray data and applied backstripping to
identify 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 frequencies
related to astronomical forcing.
This study investigates the potential added value of portable X-Ray Fluorescence (pXRF) and the application
of spectral analyses. High-resolution (cm-scale) non-destructive pXRF and natural gamma-ray measurements
were carried out on 14 m of section that was equally logged on a cm resolution. The pXRF measurements
on the surface of the outcrops are compared with earlier results of wavelength dispersive XRF spectrometry
and ICP-MS. The potassium records of the pXRF and gamma-ray logs are comparable and essentially reflect
lithological variations (i.e. between mudstone and coarse sandstones). Other reliably measured elements also
reflected lithological aspects such as clay-sandstone alternations (e.g. K, Rb, Ti), placer locations (Zr, Ce, Ti) and
potentially clay mineralogy and condensation horizons (Ni, Zn, Co, Mn).
Spectral analyses of the various proxies (lithology, natural gamma-ray and pXRF) are compared with each
other. Both the new high-resolution data (14 m of section) as well as the published low-resolution data (which
span almost 400 m of Darriwilian-Sandbian) were analyzed. The study reveals strong indications for the imprint
of obliquity, precession and eccentricity. Obtaining age constraints, in addition to the existing biostratigraphical
framework is a challenge in these sections, but would help to resolve temporal uncertainties and confirm our
interpretations. The relative strength of the potential obliquity and precession-eccentricity signals also can provide
further insights in the global glaciation history of the Middle to Late Ordovician given that a larger obliquity
component 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 Middle
to early Late Ordovician in the Armorican Massif (western France): Icehouse third-order glacio-eustatic
cycles. Palaeogeography, Palaeoclimatology, Palaeocology, 436, 96-111, doi:10.106/j.palaeo.2015.06.038