New sedimentological, petrographical, geochemical, and UPbgeochronological data from breccias of the Bas Aglysyncline (northeastern Pyrenees, France)

National audienceThis study focuses on breccias, widely developed in the Bas Agly syncline at the LateJurassic/Early Cretaceous boundary. Both their age and formation processes aredebated and remained hypothetical until today.According to our detailed sedimentological study, the breccia localized in the LateJurassic are spatially associated with large-throw normal faults and show a clearsedimentary character. They result from the creation of a steep topography thatbecomes unstable, producing major rockfalls. The studied breccias, are characterizedby poorly sorted polygenic deposits of pebbles to boulders composed of highly angularpluri-millimetric to pluri-metric carbonate clasts. A lateral evolution is observed, withpebble-size clasts found near the normal fault and boulder-size clasts away from thefault. This evolution is related to the rockfall process as the total kinetic energy acquiredby the small clasts during the fall is lower than that acquired by the bigger ones; as aresult, the latter are able to travel farther. These breccias are related to a first stage ofextension during the Late Jurassic and marks the beginning of the Early Cretaceousextension. However, the heterogeneity and complexity of the structure of carbonatebreccias require a detailed characterization of both their petrographic evolution anddiagenetic paleoenvironment in order to confirm our sedimentological model.Based on observations under S.E.M, polarized-light and fluorescence microscopy adetailed inventory of the diagenetic phases has been performed. From these analyses, adiagenetic sequence and a chronology of the cements have been reconstructed, usingprinciples of overlapping, superposition and inclusion. The oxygen and carbon isotopiccompositions of some cements and matrices of these breccias allowed to specify thediagenetic environments.The occurrence of metamorphic minerals overprinting the latest phase of calcitecementation strongly suggests that the breccia deposited prior to the Albo-Cenomanianregional metamorphic phase in contrast to previous stratigraphic attributions (Eoceneage) and support our interpretation of an interstratified sedimentary breccia of LateJurassic age. Thus, it is crucial to know the real age of these rocks. In order to solve thisissue, we initiated a geochronological study, using in-situ U-Pb dating by LA-ICP-MSand focusing on the carbonate matrix of the breccias. This approach has provensuccessful and yielded ages consistent with the proposed depositional environment