TECTONOSEDIMENTARY EVOLUTION OF THE NORTH ERN CENTRAL AND EASTERN PYRENEES DURING THE PALEOCENE AND THE EARLY EOCENE

During the Palaeocene and the lowermost Eocene, the sedimentary succes sion of the north-Pyrenzan basin. east of the Lannemezan Plateau, is m ainly formed of marine shallow water and continental formations. The b asin was bounded on the north by the Montagne Noire and on the south b y the rising Pyrenean Range, which had been growing since the Cretaceo us. From the analysis of the deposits we can demonstrate the occurrenc e of well-individualized sedimentary units which record numerous fluct uations of the marine gulf which was open toward the Atlantic ocean an d among which five major advances of the sea eastward. These units can be amalgamated into three main tectono-sedimentary units lying unconf ormably over each other in the margin of the basin. These units are: t he upper Cretaceous-Vitrolian unit, the Thanetian unit, and the Ilerdi an-Lutetian unit. The main marine advances can probably be connected t o eustatic fluctuations, such as the lower Ilerdian transgression. Som e other factors controlled the sedimentary organization of Thanetian a nd Ilerdian deposits which were due to tectonic effects at the basin s cale or even at the local scale. The stratigraphic and structural data lead us to conclude that the area of deposition of the Palaeocene and Ilerdian formations of the Pyrenees was an E-W asymmetrical syncline with an axis plunging westward. The northern boundary of this area was stable whereas the southern boundary where syntectonic formations are depositing, was active. The deformation was a shortening, the deforma tion propagating piggy-back and the structures are fault propagation f olds related to blind thrusts. The effect of the tectonic was the form ation of progressive unconformities and the migration northward of the depocentre of sedimentation