Replacements of benthic associations in a sequence stratigraphic framework, examples from Upper Ordovician of Sardinia and Lower Devonian of the Massif Armoricain

Sedimentological, palaeoecological and taphonomic analyses were carried out to establish the impact of relative sea-level variations on benthic faunas in a platform depositional context and to propose a reliable palaeoecological model for different palaeogeographic areas during different periods of the Palaeozoic. One Caradocian–Ashgillian section in Sardinia and one Lower Emsian section in the Massif Armoricain have been selected. The approach combines sequence stratigraphy, taphonomy and delineation of recurrent fossil associations. Several sedimentary facies have been recognized in relation to the distance from the shoreline. The succession consists of the stacking of genetic sequences related to sea-level changes linked to cyclic variations of orbital parameters. Several fossil associations, identified by means of multivariate analyses, are recurrent in similar facies. Eleven associations are recognized. These are the Longvillia, Nicolella, Paucicrura and Iberomena associations in the Upper Ordovician of Sardinia; the Adolfia, Stenorhynchia, Chonetid, Crinoid–Brachiopod, Ostracod–Tentaculitid, Bryozoan and Dacryoconarid associations in the Lower Devonian of the Massif Armoricain. The results of multivariate analyses based on the faunal contents are in good accordance with the clustering based on the sequence stratigraphic analysis. Benthic associations are closely related to relative sea-level variations. When the sea-level fluctuations were gradual and moderate, the association replacement is characterized by reorganization of the taxonomic composition, changes in diversity and trophic webs (association replacement by reorganization). When the sea-level changes rapidly, the association replacement is characterized by renewal of the ecological structure (association replacement by renewal). Palaeoecological changes are recognized to be strongly related to sea-level variations.