Sequence stratigraphy as an inter-disciplinary tool for karst network forecast. Multiphase and multiprocess karst genesis in the Urgonian Formation

International audienceCommonly, in carbonate reservoirs, karst systems result from long-term, multiphase and multiprocess speleogenesis that can occur from very early after the sedimentation of the limestone to presentday geomorphological events. This leads to the construction of inherently complex karst networks with a piece of architecture difficult to understand and to accurately predict. The dynamic appraisal of these reservoirs gives greater insight into the overall organization of the actual karst systems, while totally ignoring the detailed features that impact the local fluid flow behavior. In order to reduce the uncertainties in predicting the behavior of complex systems, a reconstruction of the development of the karst conduits is needed. It requires that, for each speleogenesis stage, the processes and their controlling parameters are correctly assessed.The issue of defining the prevalent parameters for the karst development is usually one on which the different specialists involved in the study of karst systems disagree. The difference in views is all the more important in the fact that specialists do not necessary address the question according to the karst origin (epigenic, hypogenic, island).For geologists, lithology and geological structure impose a general control on most cave genesis. The karst is considered to be facies-controlled, with some rock fabrics and textures described as more favorable for the development of conduits than others. Faults and fracture zones influence the secondary porosity development, through karst enhanced fractures and highly developed cavern systems. However, lithologic and structural conditions for speleogenesis evolve throughout time and change from one phase to another: progressively, the importance of fabric-selective porosity and stratigraphic elements diminishes in favor of fracture porosity.For hydrologists, hydraulic boundary conditions play as a controlling factor in karst genesis, as conduits develop mainly vertically in vadose zone and horizontally along the groundwater table, following the hydraulic gradient. Moreover, the hydraulic gradient can limit the extension or completely inhibit the development of horizontal conduits even if lateral flow path was structurally preferred. In the Urgonian Formation, a cretaceous series outcropping in south-eastern France, the development of karst conduits has been considered through geological time, and examined from many different perspectives. For several stages of speleogenesis, the relationships between various geological and hydraulic drivers with the formation of karst features have been established. An integration of most drivers has been possible through sequence stratigraphy, which influenced both the development of pore and fracture systems, the extension of seepage areas, the evolution of water table, the organization of drainage networks, and the location of inception surfaces on sequence boundaries. Sequence stratigraphy appears as an impressive tool for proposing an integrated and predictive understanding of caves development. More generally, this study highlights how much karst genesis is a multi-factorial process, which requires an inter-disciplinary approach