Influence de l'évolution paléogéographique sur les peuplements au cours du Lias dans le Moyen-Atlas Marocain

Résumé En Fr

Palaeogeographical Evolution as control of changes in fossil communities. An example from the Lias of the Moroccan Middle-Atlas. Ecological and physiographical controls on fossil communities are well evidenced in the Liassic rocks of the folded Middle-Atlas of Morocco. The palaeogeographical evolution has been controlled by the eustatic changes, which were exaggerated or lessened by local tectonic movements. The result was a highly differenciated submarine topography splitting the basin in various palaeoenvironnements in which have evolved different and characteristic communities of nectonic-nectobenthic (cephalopods) and benthic organisms (foraminifera, brachiopods, bivalves, gastropods). The described palaeoenvironments are : -inner carbonate platforms communicating rarely with the open-sea, -protected carbonate ramps and mud-flats, -neritic outer platform sustaining medium to high energy, -distal part of the outer platform with episodic waving and winnowing, -hinge-zones between slopes and basin, -large basins and highly subsiding and deepening depocenters (ombilics). Such a splitted palaeogeographical pattern have strongly controlled the evolution of organisms.

Le Lias du Moyen-Atlas plissé fournit une bonne illustration des contrôles écologiques et physiographiques sur les peuplements. Les variations eustatiques, compliquées par le jeu d'une tectonique locale de blocs, induisent une paléogéographie très diversifiée. Celle-ci détermine la répartition des paléoenvironnements dans lesquels évoluent des peuplements d'organismes nec toniques ou necto-benthiques (céphalopodes) et benthiques (foraminifères, brachiopodes, bivalves, gastropodes) à caractères bien définis.