Fluid circulation and deformational gradient in north-Pyrenean flyschs: Example from the Saint-Jean-de-Luz basin (France)

- Microthermometry in calcite and XRD analysis of clay minerals in marls were coupled.
- Fluid salinity and dispersion of salinity and crystallinity data increase downwards.
- A climate change and fluid-mineral interactions might explain the clay mineralogy.
- The deformation constrained fluid circulation and increases with the burial (< 5 km)
- An upward flow of fluid from evaporitic layers might have mixed with meteoric fluid.

The relationships between fluid circulation and deformation are one of the issues of the sedimentary basin study. In the Cretaceous flysch of the Saint-Jean-de-Luz basin, the evolution of folds geometry and the increasing volume of calcite-filled fractures and veins evidence a northward deformational gradient along the French Basque coast. A combined approach is proposed to assess the corresponding physical and chemical conditions: the microthermometric study of fluid inclusions in calcite sampled in different generations of veins and fractures and the X-ray diffraction analysis of clay minerals from adjacent marl layers. Salinity of the trapped H2O-CaCl2-NaCl fluids increases with depth in the series, in good agreement with salinity gradients reported in sedimentary basins. Dispersion of the data also increases with depth from 0.3 to 1.3 wt.% NaCl eq. in the shallowest formation (Haizabia flysch) to 9.1 to 23.0 wt.% NaCl eq. in the deepest formation (Guéthary flysch). Minimal trapping temperatures of the fluids in the Haizabia and Socoa flyschs (79 and 102 °C, respectively) are consistent with the temperatures estimated from the depth of burial, which did not exceed 5 km, in good agreement with the stability of the smectite-illite-kaolinite assemblage found in marls. In addition, the kaolinite proportion significantly decreases with depth in the series, as a potential consequence of climate changes and diagenetic transformations, whereas the increasing dispersion of illite crystallinity data might indicate fluid-mineral interactions. We propose a synthetic model of fluid circulation in the folded series that involves the mixing of mainly-horizontal fluid circulation (potentially meteoric) with an upward flow of high-salinity fluid throughout the deepest formations (potentially related to underlying evaporite-rich layers). The northward deformational gradient, as exposed along the French Basque coast, is likely to be responsible for such a vertical circulation, by increasing the volume of fracture (particularly cross-cutting fractures) in the deeply buried formations.