Résumé
This study focuses on carbonate sedimentary breccias, the so-called "Brèche limite", widely developed inthe Bas Agly syncline at the Late Jurassic/Early Cretaceous boundary. It has been clearly demonstrated in aprevious study that these sedimentary breccias of significant thickness (several tens to hundreds of meters),formed during extensional tectonics are spatially associated with large-throw normal faults. They result fromthe creation of a steep topography that becomes unstable, producing major rockfalls and eventual debrisflow. However, the heterogeneity and complexity of the structure of carbonate breccias requires a detailedcharacterization of both their petrographic evolution and diagenetic paleoenvironment.The aim of this study is to understand the origin and timing of the early mineral phases that can occlude theinterclastic space and that characterize the diagenetic paleoenvironments of the formation of these breccias.Moreover, the subaerial and/or subaqueous character of these breccias is one of the major objectives of thisstudy. For this reason, thin sections of these breccia facies were analyzed by the Alizarin – Potassium Ferricyanidemixture, cathodoluminescence, confocal and Scanning Electron Microscopy (S.E.M.). In addition,sampling of matrix and cement were carried out using a Dremel micro-drill to analyze their composition instable isotopes of carbon (δ13C) and oxygen (δ18O).Based on observations under S.E.M, polarized-light and fluorescence microscopy a detailed inventory of thediagenetic phases has been performed. From these analyses, a diagenetic sequence and a chronology of thecements be reconstructed, by using principles of overlapping, superposition, and inclusion. The occurrenceof metamorphic minerals overprinting the latest phase of calcite cementation strongly suggests that thebreccia deposited prior to the Albo-Cenomanian regional metamorphic phase and support our interpretationof an interstratified sedimentary breccia of Late Jurassic age.Finally, the present work provides a set of petrographic criteria which allows discriminating the nature ofcarbonate breccia (sedimentary vs tectonic) and interpreting the physical processes leading to their formationand their depositional environment.Keywords: breccias, sedimentary, extension, diagenesis, geochemistry, S.E.M, Late Jurassic, Pyrenees.