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Impact des variations eustatiques sur les assemblages benthiques à Brachiopodes : l'Ordovicien Sarde et le Devonien Ibero-Armoricain

Abstract : The aim of the present work is to study the relationships between Palaeozoic benthic organisms and palaeoenvironmental changes. Sedimentologic, palaeoecologic and taphonomic analyses have been carried out to establish the impact of relative sea level
variations on benthic faunas, in a platform depositional context and to propose a reliable palaeoecologic model for different palaeogeographic areas during different periods of the Palaeozoic.
One Caradoc-Ashgill stratigraphic section in Sardinia, five Lower Emsianstratigraphic sections in Armorican Massif and a Lower Emsian stratigraphic section in Spain (León) have been selected. The approach consists in applying simultaneously sequence stratigraphy and palaeontological studies (taphonomy, delineation of recurrent associations, systematics). Only the brachiopods are described and illustrated in this work. The brachiopod fauna includes 20 species from the Upper Ordovician of Sardinia and 33 species from the
Lower Devonian of the Ibero-Armorican domain.
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. This signal resulting from changes in relative sea level is linked to the cyclic variation of orbital parameters.
Several associations, identified by means of multivariate analyses, are recurrent in similar facies. Sixteen associations are recognized. These are the 'Longvillia', 'Nicolella', 'Paucicrura' and 'Iberomena' associations in the Upper Ordovician of Sardinia; the 'Adolfia', 'Stenorhynchia', 'Chonetids','Crinoids-Brachiopods', 'Crinoids-Ostracods', 'Ostracods', 'Ostracods-Tentaculitids', 'Bryozoans' and 'Dacryoconarids' associations in the Lower
Devonian of the Armorican Massif; the 'Rhynchonellids', 'Spiriferids' and 'Tetracorals-Crinoids-Dacryoconarids' associations in the Lower Devonian of Spain. A large part of the
fossil record is preserved in shellbeds. The study of shellbeds, taking into account taphonomic signatures and their repartition at the scale of a genetic sequence, allows us to distinguish 2
types of shellbeds and to suggest a genetic model for their formation. Type A shellbeds are associated with high or moderate supply episodes during sea level fall, while type B is related
to low supply episodes during sea level rise. These episodes are characterized by the stability of the water-sediment interface, allowing taphonomic feedback and activity of boring organisms (bioerosion). The resulting shellbeds consist of condensed layers distinguished by the amalgamation of several generations of organisms.
Coupled palaeontological and sedimentological studies are efficient tools to understand the association dynamics. The results of multivariate analyses based on the faunal contents is in good accordance with the clustering based on the sequence analysis.
Associations are closely related to relative sea level variations. When the sea level fluctuations are gradual and moderate, the association replacement is characterized by the reorganization of the taxonomic composition, the diversity and the trophic webs (association replacement by reorganization). When the sea level changes rapidly, the association replacement is characterized by the renewal of the ecology structure (association replacement
by renewal). Palaeoecologic changes are recognized to be strongly related to sea level variations.
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Arnaud Botquelen. Impact des variations eustatiques sur les assemblages benthiques à Brachiopodes : l'Ordovicien Sarde et le Devonien Ibero-Armoricain. Minéralogie. Université de Bretagne occidentale - Brest, 2003. Français. ⟨tel-00007123⟩

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