Sea-level curve for the Middle to early Late Ordovician in the Armorican Massif (western France): Icehouse third-order glacio-eustatic cycles

This contribution deals with the Middle to early Late Ordovician of the Armorican Massif (western France). The aim is to reconstruct a multi-order eustatic curve from the stratigraphic record. The studied section (Crozon Peninsula area) shows sediments deposited in tidal to storm-dominated shelf environments. A sequence analysis (facies and stacking pattern analyses), combined with a gamma-ray record and a backstripping procedure, was carried out. Chitinozoan assemblages provide the biostratigraphic framework including time calibration. The stratigraphic succession is dominated by the stacking of sequences representing different orders of sea-level change i.e. very high- (b400 kyr), high- (400 kyr) and low- (3rd order) frequencies. Atime-calibratedsea-level high frequency curve, corrected for compaction, sediment load and tectonic subsidence, has been calculated based on a backstripping procedure. The smoothing of this curve permits identification of eleven thirdordersequences with seven episodes of significant (50 to 80 m) and rapid (less than 100 kyr) sea-level falls during the Darriwilian and the Sandbian, and two major transgressive events located in the formosa and pissotensis biozones of the Darriwilian. The reconstructed sea-level curve reveals thatmost of the third-order cycles comprise three fourth-order cycles of assumed 400 kyr duration. This 1.2 myr duration of the Ordovician thirdordersequences suggests that they may be related to long-period obliquity cycles, an orbital forcing characteristic of icehouse sequences in the late Cenozoic and the Quaternary. These results imply that themajor part of theMiddle and early Late Ordovician was characterized by icehouse conditions and third-orderglacio-eustatic cycles. A time calibration of the chitinozoan biozones was developed from the sequential analysis. It suggests that the early Darriwilian biozones (from henryi to calix biozones) have short durations of several 10 to several 100 kyr whereas the pissotensis Biozone has a temporal extent of several myr, extending into the early Sandbian.