Signification écologique des foraminifères acervulinidés et leur rôle dans la formation de faciès récifaux et organogènes depuis le Paléocène
Résumé
Acervulinid encrusting Foraminifera are rarely reported in present day reef studies of reefs and have never beenused as ecologic indicators in fossil reef reconstructions. Comparative sedimentological and ecological analysis of acervulinid facies in reefs and other biogenic formations of various ages and geographical locations (early Eocene Pyrenean region, south France and north spain, Plio-Quaternary Loyalty Islands, northeast New Caledonia and Quaternary Mururoa atoll, French Polynesia) demonstrates their role in reef construction since the Paleocene and their ecological significance in relation to their growth forms. In fossil and recent reefs, the biological framework built by acervulinid Foraminifera is best developed in environments where ecological conditions lead to the reduction of competition for substrate encrustation. These ecological factors may have different origins but are frequently related to a decrease of light intensity, light deficiency being a discriminating factor between encrusting Foraminifera and major reef building organisms like hermatypic corals and red algae. The contribution of acervulinid Foraminifera to organic build-ups is generally limited in shallow, external environments of reefs, where the acervulinids are associated with various other encrusting organisms. On the other hand, these Foraminifera become a major constituent of the reef framework or in formations made of macroids where ecological factors involve the decrease of competitor organisms. During early Eocene times, in the Pyrenean region, acervulinids were able to build monospecific kilometre sized reefs, the excessive development of these original build-ups being probably favoured by local conditions and especially the subtropical climate.