Thesis defense of Alexandre Ortiz

Geometry and source-to-sink of a retro-foreland basin during its late to post-orogenic evolution : the case example of the Pyrenees / Aquitaine Basin / Bay of Biscay from 38 to 0 Ma

Abstract

The purpose of this thesis is to study the "source-to-sink" system of the Pyrenees system and its retroforeland basins, the Aquitaine basin and its deep equivalent, the Bay of Biscay during the Cenozoic. This work required (1) a biostratigraphic re-evaluation, (2) an analysis in terms of seismic stratigraphy and quantification of preserved sediment volumes, (3) a quantification of eroded volumes from the Massif Central, (4) a quantification of the eroded volumes from the Pyrenees, (5) a synthesis of all these data. The transition from the orogenic to the post-orogenic phase occurs between 27.1 and 25.2 Ma. It corresponds to the succession of three erosional surfaces that fossilize the entire compressive structures. The orogenic period is divided into two phases, (1) up to 43.5 Ma (Lutetian), is characterized by a strong subsidence at the front of the North-Pyrenean-Thrust, (2) from 43.5 to 27.1 Ma, is characterized by the subsidence migration toward the basin, in sub-basins controlled by the thrusts activity. The post-orogenic period is divided into two phases, (1) from 25.2 to 16 Ma approximately, corresponds to the establishment of the isostatic rebound, (2) between 16 and 10.6 Ma, corresponds to an uplift of the whole system. This phase corresponds to a West European event undoubtedly linked to a mantle activity. The total quantity of rocks preserved in the Aquitaine basin and the Bay of Biscay is 92 200 km3. The distribution of sediments preserved over time evolves in favor of the Aquitaine basin between 66.0 and 33.9 Ma and in favor of the Bay of Biscay between 5.3 and 0 Ma. This balance is due to the different stages of evolution of the subsidence / uplift in the Aquitaine basin. The sedimentation rates show two periods of increase in sedimentary fluxes, the first at the Eocene-Oligocene limit, which we relate to both the period of Pyrenean paroxysmal exhumation and to contemporary global cooling. The second, at 5.3 Ma, seems to correspond to the global increase of fluxes, whose climatic origin is favored by the authors. The total amount of eroded rock from the Pyrenees and the Massif Central is 34 335 km3. The difference observed between the sedimented volumes and the eroded volumes can be explained by the contribution of sediments resulting from the currents from the Pliocene, the not taking into account the volumes coming from the Cantabrian massifs and an underestimation of the eroded volumes.