Diagenesis of carbonate sedimentary breccias formed during Late Jurassic extension, Bas Agly syncline (Pyrenees, France)
Résumé
This study focuses on carbonate sedimentary breccias, the so-called "Brèche limite", widely developed in
the Bas Agly syncline at the Late Jurassic/Early Cretaceous boundary. It has been clearly demonstrated in a
previous 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 from
the creation of a steep topography that becomes unstable, producing major rockfalls and eventual debris
flow. However, the heterogeneity and complexity of the structure of carbonate breccias requires a detailed
characterization 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 the
interclastic 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 this
study. For this reason, thin sections of these breccia facies were analyzed by the Alizarin – Potassium Ferricyanide
mixture, 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 in
stable isotopes of carbon (δ13C) and oxygen (δ18O).
Based on observations under S.E.M, polarized-light and fluorescence microscopy a detailed inventory of the
diagenetic phases has been performed. From these analyses, a diagenetic sequence and a chronology of the
cements be reconstructed, by using principles of overlapping, superposition, and inclusion. The occurrence
of metamorphic minerals overprinting the latest phase of calcite cementation strongly suggests that the
breccia deposited prior to the Albo-Cenomanian regional metamorphic phase and support our interpretation
of an interstratified sedimentary breccia of Late Jurassic age.
Finally, the present work provides a set of petrographic criteria which allows discriminating the nature of
carbonate breccia (sedimentary vs tectonic) and interpreting the physical processes leading to their formation
and their depositional environment.
Keywords: breccias, sedimentary, extension, diagenesis, geochemistry, S.E.M, Late Jurassic, Pyrenees.