New sedimentological, petrographical, geochemical, and UPb geochronological data from breccias of the Bas Agly syncline (northeastern Pyrenees, France)
Abstract
This study focuses on breccias, widely developed in the Bas Agly syncline at the Late
Jurassic/Early Cretaceous boundary. Both their age and formation processes are
debated and remained hypothetical until today.
According to our detailed sedimentological study, the breccia localized in the Late
Jurassic are spatially associated with large-throw normal faults and show a clear
sedimentary character. They result from the creation of a steep topography that
becomes unstable, producing major rockfalls. The studied breccias, are characterized
by poorly sorted polygenic deposits of pebbles to boulders composed of highly angular
pluri-millimetric to pluri-metric carbonate clasts. A lateral evolution is observed, with
pebble-size clasts found near the normal fault and boulder-size clasts away from the
fault. This evolution is related to the rockfall process as the total kinetic energy acquired
by the small clasts during the fall is lower than that acquired by the bigger ones; as a
result, the latter are able to travel farther. These breccias are related to a first stage of
extension during the Late Jurassic and marks the beginning of the Early Cretaceous
extension. However, the heterogeneity and complexity of the structure of carbonate
breccias require a detailed characterization of both their petrographic evolution and
diagenetic paleoenvironment in order to confirm our sedimentological model.
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 have been reconstructed, using
principles of overlapping, superposition and inclusion. The oxygen and carbon isotopic
compositions of some cements and matrices of these breccias allowed to specify the
diagenetic environments.
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 in contrast to previous stratigraphic attributions (Eocene
age) and support our interpretation of an interstratified sedimentary breccia of Late
Jurassic age. Thus, it is crucial to know the real age of these rocks. In order to solve this
issue, we initiated a geochronological study, using in-situ U-Pb dating by LA-ICP-MS
and focusing on the carbonate matrix of the breccias. This approach has proven
successful and yielded ages consistent with the proposed depositional environment.